WHAT IS IT?  A concentration, frequently used for the volume/volume concentration of a contaminant in air.

E.g. 600 L chloroform vapor in an 8,000,000 L room...

(600L)/(8,000,000L) = 0.000075 = 75 ppm
(divide and then move the decimal place to the right, just like for percent except move it 6 instead of 2 places)