The inseam of a pair of pants refers to the seam that runs along the inside part of the leg. Inseam is also used to refer to the length of the pair of pants from the crotch down to the cuff (the length of the inseam).
When referring to the length of a pair of pants, the inseam is usually the number that is used. It can be helpful when shopping for pants to know the length of your inseam, so you get the pants of appropriate length.
Generally, your pants should be long enough to cover the top portion of your shoe, though it depends on the type of pants and also on the type of shoes you are wearing. Measuring your inseam is simple, just take a tape measure and measure from the center of your crotch down to your ankle.