Not sure if it is still useful info for you or not since you have probably long since purchased a pair of new pants
Figured I'd give a quicky answer though just to clarify and keep this thread up to speed.
PSI stands for Pounds per Square Inch just like it did in your high school physics class...basically it is an indication of how much pressure the garment can withstand before water seeps in...in this case the garment can withstand 25 pounds of pressure per inch of fabric before moisture seeps in. The average guy exerts about 16 pounds of pressure per square inch kneeling on the ground. The fabric is tested in what is refered to as the Mullen's Test and is one of the industry standards but is slightly different than the Water Column Test in which a tube of water, 1 inch in diameter is filled with water to test how many millimeters of water the fabric can withstand before leaking over a 24 hour period of time. Water Column Test values are shown in millimeters (mm).
To compare mm rated fabrics to PSI rated fabrics keep in mind that 576.834 mm = 1 PSI so a 25 PSI rated fabric is approximately equivalent to a 15,000 mm jacket (give or take 570.15 mm)
Hope this helps!