what is a challenge run?

A challenge run is a broad term for a playthrough done with a specific goal/set of restrictions that make the playthrough more challenging. "Beat the game without taking any damage" or "beat the game without using certain actions/abilities" are common examples. They can be done as a speedrun, but they don't have to be (technically, a speedrun is also a type of challenge run, but people are usually not referring to speedruns when they talk about challenge runs).

Any particular reason you're asking? Without any context it's hard to tell if this is just idle curiosity or if it's part 1 of a larger question.

@ShikenNuggets he’s probably asking because of the new Ghostrunner 2 Challenge

I don't know why you @'d me instead of just answering the question then, but anyway, if that is what this is about then here:

thanks!