Those letters indicate the type and quality of signal detected by your BlackBerry.
1x - digital signal is present but is not of good quality, you will not be able to use data (and often voice) services here
1X - good quality digital signal
1XEV - EvDO signal, broadband data speeds are possible here
Give this a read if you want to get a little more technical: http://en.wikipedia.org/wiki/1x