it's a little tsu (tsu being the sound if it is not little - observe ツ compared to ッ), essentially a glotteral stop (or if you want to get really technical, it is a geminate consonant)
It has no meaning by itself.
It is used to indicate a slight pause in between two sounds in Japanese. For instance, if you say the word 'rock' (or how I say it anyway) you will find a pause between 'ro' and 'ck' (if you say it without the pause, it will sound more like 'rawk'). You probably wont hear it though, as in English we don't really need to use it to make a distinction between words that sound similar, and therefore don't mark it, and therefor don't hear it till we are trained to notice it.
In Japanese it can make the difference between words, for instance:
キタ kita, I came
キッタ kitta, I cut it
They sound slightly different, the latter having a slight pause (so to speak) between ki and ta
I wouldn't agree with bellie, that is shortens the vowel sound much at all (as you can say these things as slowly as you want really), it is more a lengthening of the gap between voicing one sound and voicing the next. Or more of a sudden stop