Mods are programs that add new features to MCPE.
For example, MCPE could have a gun mod (which adds guns to the game) or an ammo mod, or a zoom mod, etc.
There could also be lower-level mods, such as a screenshot mod (which adds a screenshot button), a jump-higher mod, etc.
NOTE: A mod is not a glitch – a glitch is a bug in the original MCPE game. However, a mod is the product of hacking into the MCPE files.