What does to touch on mean like this example?
a interesting topic touching on everything you have mentioned

Question

Answer

I'd understand it as 'referring to'.

Answer

the context is missing, so it could be one of these example, or a similar variation:

an interesting topic - which is relevant to - everything you have mentioned
an interesting topic - which refers to -  everything you have mentioned
an interesting topic - which is linked to - everything you have mentioned