This practice is commonly used by less-than-trustworthy suppliers in order to boost revenues.
Cuts used in powdered, crushed, sharded, or 'rock' substances can include: sugar, flour & salt. There are many more, but these are the common ones. The afore mentioned cuts are sometimes added in the making of the substance, but can also be added post-production. A common process for the shifty crystal meth dealer is to measure out a small amount less than you're buying and add a couple of drops of olive or vegetable oil to your baggy to make up the remainder of the weight.
In cut substances that one smokes, the best reccomendation is to melt the substance pre-smoking, and continue to apply heat in 8-10 second increments until the cut has burned off. You will know the cut has burned off because, when the heat is removed, the substance will re-form quickly. (Cut substances generally remain liquidy/runny much longer than the true substance should) In burning off the cut, be cautious not to burn the substance. If the substance is cut with sugar, it is more likely to burn easily, so be wary. Oil cuts can make the substance pop or crackle when exposed to heat.
example #2: imma jus cut mrs. smiths class cus I failed her class already.
example #3: damn, the school called my house today cus i've been cutting school for 2 weeks
person #1: i didn't see you in class yesterday
person #2: cus i was cutting