What is it mean Difficuly & prob when using vanitygen?
Difficulty tells you how likely it is that vanitygen will be able to find the address pattern you're looking for on the next guess. A value of 2n is twice as difficult (half as likely) than a value of n.
Because the process is completely random, it's impossible to give you a '100%' ETA. We can only estimate that, on average, you will find your key before some time period half the time (hence why 50% is used). Note that it re-adjusts the value to 75% once it hits that 50% mark (and re-adjusts again after hitting 75%, until the key is eventually found).
That is the likelihood that you would have found your key by now, from the time when you started generating your key.