This is a well-known set phrase in English. It means that you should ask for something that you want.

Use "It doesn't hurt to ask" when you think that someone should ask for something, even if the answer is probably "no". Here is another example:

I doubt that he knows where it is, but it doesn't hurt to ask.

You can also use this similar phrase:

There's no harm in asking.

What if you think that asking for something might have good results but on the other hand might cause problems? Then you can say something like this:

It might be worth asking if they'll consider giving you a raise.