The metro is the local train system. You can take the metro from the airport to metro center (yellow line) and then go to union station from there on the red line. It’s maybe a 30 min trip depending on train service.

Plenty of info online (including a map) but people will be happy to help you figure out the metro when you get to the airport if you need help. Every station has a station manager and the airport especially has tons of people who don’t usually use the metro so they’re used to it. You can download the WMATA smartrip app beforehand to speed things up - it’s a card on your phone that you can load money onto.