P stands for progressive and I is for interlaced. Progressive will take each line of pixles and refresh them (so ln 1,2,3,4...) Interlaced will do every two and then do the other lines on the next pass (so 1,3,5,7,9... then 2,4,6,8,10...) It will take Interlaced 2 passes to do what Progressive does in 1 pass.
What that really means is that when your screen refreshes at say 60Hz progressive will refresh all of your lines of pixles 60 times a second. Interlaced will refresh all of your pixles 30 times a second.