It’s a very, very small gene pool of very, very closely associated people.

Their risk factor is much higher than other groups.

Tay-Sachs is famously much more prevalent among Ashkenazi Jews, not because it cares about your religion, but because it's a rare recessive disorder, so both of your parents have to happen to be carriers to pass it on. As a result, it will naturally be more prevalent in any smaller interbreeding population where it happens to become slightly more prevalent by chance. Historically, due both to internal and external pressure (i.e. religious encouragement to marry other Jewish persons as well as generations of Jews being cut off as outsiders from the majority populations in many places where the Ashkenazim have lived), Ashkenazim have been more likely to marry and reproduce with other members of the same closed group than with the wider population, so chance occurrence of Tay-Sachs somewhat above the norm dramatically increases the (still small, but the disease is so terrifying it's worth preparing around) chance of it appearing in offspring.

It's not only an "Ashkenazi disease"; other relatively closed groups have higher incidences, like French Canadians and Cajuns, and some Amish groups.

If the reference to Ashkenazim is what makes it sound remarkably specific, be aware that this simply means more or less "Jews of Germany" and in practice means even more broadly the Jewish diaspora population of most of Northern, Central, and Eastern Europe; the Jews whose ancestors likely spoke Yiddish -- as opposed to, say, the Jewish populations of Iberia, the Sepharadim, or the Mizrahim in the rest of the Mediterranean, sometimes grouped as part of the Sepharadim. As such, it means a very broad swath of the Jewish population, and in particular a group to which most Jews in the US belong, since so many emigrated from Central and Eastern Europe to the United States.

ETA: I worded the 2nd paragraph under the mistaken recollection that the Cajun population had inherited their propensity from French Canadian ancestors, but a bit of reading just now says that apparently the two are demonstrably not related, since the genetic mutations among the Canadian and Cajun groups are different. Apologies for being misleading, but that means that they should be considered two distinct semi-closed populations where prevalence is higher.

Intake paperwork for obstetrician’s offices specifically ask if either parent is an ashkenazi Jew. It’s a specific concern for that particular race due to a genetic bottle neck in their ancestral population.

The Jewish people have what we (Jews) call “close knit communities” which result in “intercommunal breeding”. It’s not nearly as bad as it sounds, but it does increase the concentration of certain genetic traits. You can see examples of this in purebred animals. Pugs, for example, didn’t always used to look like their faces got smooshed up against the window and as a result of breeding that specific trait, nearly all purebred pugs have serious health issues, especially when it comes to breathing.