I am taking an introductory Topology course. I am having a completely impossible time with it.

If somebody could explain in simple terms what this means, I would greatly appreciate it:

Is an open subset of with respect to the Sorgenfrey topology?

Provide a proof for your answer.

(I am not trying to get you to do my homework. This is a question from a previous exam, and I am just trying to figure out how to do these questions).