Audio normalization(?) for mp3 files

Let's start this right: I AM NOT A TECH. Let me repeat that: I AM NOT A TECH.

I have a batch of files that have 50 or so mp3 music files in each file.

The mp3s come from different sources, and were obviously recorded at  different levels.  If you were listening to one file of say, a dozen mp3s (songs), you might need to turn the volume up or down for various ones.

By the way, did I mention that I am NOT a tech?

What I'd like to do is to adjust the levels of all the mp3s in all the files so they're all about the same, eliminating the need to "turn the volume up--or down." I'd like to do this using software that can do the adjusting for all songs in one file in "a batch" so that I do not have to do each one individually.

And in case i didn't mention it, I am not a tech!

I have done a lot of online reading about audio equalization and normalization and have come away without any real understanding because---can you guess?--I AM NOT A TECH.

The only thing that made a LITTLE sense was that MAYBE MP3 gain was what I should use. But I'm not sure.

So here I am, hoping that an expert who can explain something to someone who is NOT A TECH in plain English, will help me out.

Any takers?