总有 一个 在 路上 what does 在路上 mean here?

总有 一个 在 路上
what does 在路上 mean here?

Literally it should be on the road, or on a trip. 路 is road (etc) and sticking 上 (on) at the end basically makes a new noun 路上.

Yes, this just means “on the road”.

but why add a 在 if 路上 already means ‘on the road’, is the 在 there to indicate to be physically ‘on the road’?

路上 sort of means “on the road” but it is basically a noun referring to a place where something could be. So yes, you need 在 to bring it into the sentence in a grammatical way.