Digestion is the process in which large molecules are hydrolysed by enzymes to produce smaller molecules that can be absorbed and assimilated.

Hydrolysis is when a molecule is broken because water has been added to its chemical bonds:
Amylase breaks starch down into maltose;
Lipase breaks lipids down into glycerol and fatty acids;
Protease breaks proteins down into amino acids.

Absorption is when broken down food molecules diffuse into the blood stream.
Assimilation is when the broken down food molecules are taken up by cells (so that they can be used by them.)