GT3 Rip Guide

[laser_wgn]

I am ripping GT3 so i got the guide Gran Turismo 3 A-Spec Ripping Tutorial (Pal and USA) By Morganator
i dont understand Step 3

Step 3, Go into the folder called stream and downsample the following files while deleting the remaining ones.

course.ipic
apricot_full.pss
akasaka_full.pss
deepforest_full.pss
the list continues with lots of .pss files

Wat does it mean?