Hum and Guitar String Ground

Hum and Guitar String Ground

So here's the problem: 

You’ve got your guitar plugged in. You’re not playing it and there’s a noise. A hum. It’s not terribly pleasant. 

Thing is, you touch the strings and it’s gone.

The response: It's ok. It's not a problem. That’s all as it should be. Guitars pick up interference and that comes out the amp as a hum. When you touch the strings, it's supposed to get quieter. 

Because, 'grounding'.