This refers to underwear in general. To be more specific, say rejghun yIvbeH, rejghun yopwaH bID, etc.

qepHom 2022:

LL: What is the plural form of rejghun, "underwear", if anything at all?

MO: rejghun is a mass noun with no singular counterpart. So you can't say "undergarment"; you have to say "undershirt," etc. If you're referring to a collection including shirts, shorts, etc., or if you don't know what's in that pile on the floor, rejghun alone is okay. 

LL: So how does one say "undershirt" etc? Are there words for those, or do I say things like rejghun yopwaH and rejghun yIvbeH?

MO: You just say rejghun yopwaH and so on.

Though there are many styles of shirts, a rejghun yIvbeH is stereotypically a particular style (or a couple of particular styles), so the type of yIvbeH doesn't have to be mentioned (unless it's an unusual sort of undershirt or being contrasted with another type). You probably wouldn't say rejghun yIvbeH SeQHa' unless contrasting it with a shirt with long sleeves. 

Similarly, though yopwaH normally refers to long pants, rejghun yopwaH are typically shorts. You could, but don't have to, say rejghun yopwaH bID. The usual way to refer to long johns (underpants with long legs), when a distinction is necessary, is rejghun yopwaH naQ.

Note that in the voSpegh Sep dialect, in which yIvbeH generally refers to a shirt with no sleeves, a rejghun yIvbeH may or may not have sleeves.

When the context is clear - if the conversation is about underwear, for example - you can leave the rejghun off.

I know this will lead to questions about particular underwear (and other) styles, but Maltz is not a clothes horse.