X stands for errors (jitter errors) while ripping the track. The number is the no. of such errors. You should also see the no. of errors accumulating while the track is being ripped in the little window, if you're around while ripping.
The presence of errors doesn't necessarily mean that the errors are noticeable when you play the mp3 track (I find most of the time they aren't, unless the error no. is really large - but the bad ones from poor CDs can "skip" really badly).
You can play the mp3 file through to check, or simply re-rip the track (which is quicker) to see if the errors repeat.
Often the errors disappear on a second attempt.
Try not to use any other application if you have a problematic CD.
Errors can be caused by a number of things. Dirty CD, or simply old CD (CDs don't last as long as we think they should). Try cleaning the CD.
Try another CD drive if you have one. Different CD drives produce different read errors.
You can also change a few settings to see if it makes any difference. Click Options in the top menu, Settings, CD Drive tab.
Make sure that "Enable Jitter Correction" is ticked, which by default is. [If your CD and CD drive are good, you can untick this to have a faster rip.]
Set "CD Speed" from 0 to 1. This will slow down the ripping process but may get you a better result with a bad drive or CD. [Remember to change it back to 0 afterwards.]
Set "Read overlap" to a larger number, but never larger than the number in "Read Sectors" just above it. Again change it back to 7 afterwards.
Click OK to change the settings. Re-rip. If the errors still repeat, there's nothing you can do except perhaps try another software and hope you're lucky, but don't give up on CDex.
You can also explore the various tabs in Settings which give you a lot of control over what you want to do.