Question:   What is an activity series, and how can it help to predict how metals will react with acids?

Answer:

The whole thing about reactivity series is that they tell you how reactive elements are going to be.  For example, if metal X will react with compound A but metal Y won't, we just say it's higher in the reactivity series.

This comes in handy because let's say we have another metal, metal Z.  If we notice that metal Y reacts with compound B but metal Z doesn't, metal Z is lower on the activity series. However, this also tells us a couple of other things about how metal Z is likely to react.  It tells us that if we go back to compound A (from the first paragraph), metal Z will not react with it because it's even less reactive than metal Y.

As far as predicting how metals react with acids, the reason that you wouldn't want to put a reactive metal in contact with acid is just that metals generally react pretty well with acids, and the higher the metal is on the activity series, the more likely it is to react with an acid.