The pant inseam is technically the measurement from the seam at your "crotch area" (for lack of a better term) down to the bottom of your pants (near your foot/ankle).  (NOTE: for reference, your outseam is the term used for the measurement from your top hip down to the bottom of your pants.  Some tuxedo rental stores prefer to use :outseam" measurement -- mostly because I don't think they want to put their tape measure near your "goodies" ... :)

 

When you go to a store and buy a pair of jeans, you normally buy them based on your "waist size" and your "pant inseam".  For example, maybe you get your jeans in a 34 waist / 32 inseam ... or maybe a 38 waist / 30 inseam, etc.

 

With most tuxedos, the pants come "unhemmed"; however, you are given plenty of extra fabric to be on the safe side.  For example, you will have 37+ inches of fabric on all tuxedo pants (whether you select a "Short rise inseam ... 27 - 29", "Regular rise inseam ... 30 - 32" or "Long rise inseam ... 33 - 35", etc.  In general, think of "rise" associated with height.  If you are a short guy, go with the short rise; if you are a regular size guy, go with the regular rise; if you are tall guy, go with the long rise; if you are a really, really tall guy, go with the X-Long rise.

 

In case you were wondering on why all that extra fabric?  Manufacturers provide this extra fabric in case you want to "cuff" your pants; however, it is recommended that you do a "straight hem" with tuxedo trousers, not a cuff.