it's basically two lists linked together
you have a "key" and to that key you assign a "value"
if for example you wanted to store points in a dictionary
the "key" would be the players username (or uuid) and the value would be the points
need to retrieve the points later? you simply use "get value from key" or whatever it's called I don't remember
the slight difficult thing is when you're trying to get a key from a value, in which a tutorial probably would be needed, but it's not that often that you need to do that
hope this helps