What does the slash mean in effects like "Push up to 3 (Explorer)/(Town)"?

What does the slash mean in effects like "Push up to 3 (Explorer)/(Town)"?

It means "pieces of either or both types" - you are Pushing up to 3 pieces; those pieces may be Explorers and/or Towns.
Similarly, "1 Damage to each [Town]/[City]" means "1 Damage to each piece that is either a Town or a City". (Or, in theory, both - though this would require some bizarre new piece considered both a Town and a City in order to be a relevant case.)
This is described in the reference on the back cover of the rulebook.
(Formally, the slash is a union operator.)