The number represents how many vertical lines are on the screen, the p stands for progressive scan, meaning non-interlaced.
Basically, higher numbers mean there are more spaces on the tv to create the picture, resulting in a better quality picture.
an interlaced screen is good in that it decreases bandwidth, but has a tendency to create flickers on the screen, kind of like this.
I forget how progressive scan works off of the top of my head, I just know it is good. A quick google search will tell ya what it does.