The word "gerrymander" comes from a political cartoon in the Boston-Gazette from 1812. The paper was criticizing then-Governor Elbridge Gerry (1744-1814) for using his influence to get electoral maps in Massachusetts drawn in a politically favorable way for him, but that defied reasonable expectations of fairness. One district in these maps vaguely resembled a salamander, so the Boston-Gazette added some artistic flair and dubbed it the "Gerry-mander". It is important to note that Gov. Gerry did not invent the process of gerrymandering, nor was he even necessarily the worst offender up to that time. Rather, the Gerry-mander was just catchy enough of a name that it caught on for the whole process for the rest of time.

So what is gerrymandering? As I said before, it's drawing electoral maps in a politically advantageous way for a particular politician or faction. But to really understand what that means you probably need an example. So the first thing you must understand is that not every person has the same ideals and interests, and will therefore tend to vote in different ways. You might be a rich factory owner interested in tax breaks and relaxed labor laws; I might be a subsistence farmer interested in keeping property taxes low and a relatively clean environment so that my crops don't fail or poison me. Joe Shmoe over there might be a city-dwelling white collar worker interested in urbanism and robust government services like education and city parks. Further, these people with differing interests are not necessarily homogeneously spread throughout a particular polity. The factory owner and white collar workers mentioned above are more likely to reside in a dense city, while the subsistence farmer is more likely to be out in the country, for example. Indeed, if you look at the current American political landscape you can see that dense urban areas tend to be heavily Democratic voters while the surrounding exurbs and rural areas tend to vote heavily for Republicans.

So with those two factors in hand, a savvy politician tasked with making maps for the next decade until the next decennial census might be able to predict how individuals in any given are are likely to vote, and then draw boundaries in such a way to achieve some political goal. Generally, this falls under two major methods, "cracking" and "packing". In cracking, map makers will attempt to split groups up in and pair them with their opposites in such a way as to dilute the target group's influence. For example, a Democratic-leaning city might be split into two or more districts and paired with the less dense, but more populous, Republican-leaning countryside areas in such a way as to produce a slight but reliable Republican majority in each district, effectively eliminating the ability of the people in the city to have a voice at the particular level of government at question.

Packing is slightly different, in that you try to pack as many target voters into a single district so that their electoral efficiency is low. This will produce a seat for the opposition, but if that seat is a minority, it's fine for the gerrymandered because it will be made up for with electoral gains elsewhere. By electoral efficiency, I mean that for a single member district, it doesn't really matter if that member wins with 51% or 99% of the vote; they're still just a single vote in the legislature. So a Democrat might try to pack as many reliable rural Republican voters into as few districts as possible, making just a few deeply entrenched Republican seats in the legislature, then use the rest of the of the map and the seats to deliver a solid and everlasting Democratic majority.

Either way, the end result is politicians choosing their voters, rather than voters choosing their politicians. Both parties do it. Texas engaged in quite a bit of packing this cycle to further cement their Republican majority with their two new congressional seats, while Oregon did a bit of cracking of the rural vote in an attempt to expand their majority with their new congressional seat. Most of the time, you have to do a bit of both to pull off a truly successful gerrymander. Also, gerrymandering isn't always done for a party's benefit; sometimes incumbent politicians will want to draw nice safe seats for themselves, even if they end up in the minority most of the time. After all, it's a pretty nice gig to cruise to an easy reelection every year and get to sit in the halls of power without ever actually having to the hard work of governing and facing the consequences of your policy positions being enacted. Also, gerrymandering isn't always done for partisan purposes. For example, in the US we have a requirement to create majority minority districts when possible in order to ensure that minorities voices can be heard in government. So in the Deep South you'll often see packing of the Black vote into one or two sapphire blue districts so that Black voters can know that they'll have someone they've chosen for themselves on Capitol Hill. Whether that's a good or bad thing is a value judgement left as an exercise for the reader.