what do these numbers next to the carriers mean?

Pretty much a more technical way to see the connection strength. The higher the number (closer to 0), the better your connection.

You can see it by going to the phone app and dialing the following exactly. This will launch a field test app where you can see a bunch of technical info, and also replace the bars with the number. Once you quit out of it, it will go back to normal.
*3001#12345#*

 

Are you talking about the numbers that are where the signal strength meter should be? That's a very ugly tweak you can get if you're jailbroken.

 

Funny that you asked directly after someone else asked how to get them. Those numbers indicate the cellular signal strength in db. More info here.

 

*3001#12345#*
call that number, turn off your phone or force quit the app, turn phone back on, congrats you have numbers now.