I bet you know that the prefix "non-" means "not." Combine that with the word "descript," which means "easy to describe because of parts that are easy to notice," and you get "nondescript."
Nondescript things and places are hard to describe, because they're boring, they're not interesting, or they don't have any pieces that catch your eye.
For example, a nondescript ant doesn't look special in any way; it just looks like any other ant. It's hard to describe it because it doesn't have any distinct characteristics.
And a nondescript town has no particular landmarks or quirks. There's nothing special about it, nothing that helps you describe it or remember it after you visit.