What is absorption?

Absorption is a process in which a substance is assimilated in another substance. The substance that gets absorbed is known as the absorbate and the bulk phase in which the absorption takes place is known as the absorbent.
For example, when we use a paper towel to clean up a water spill, the water is absorbed by the paper. In this example, the paper towel is the absorbent and water is the absorbate.
One can find a number of examples of absorption in daily life. Plants absorb water from the soil, and small intestines act as the site for nutrient absorption in our body. 
Kindly note that absorption is different from adsorption. The process of adsorption includes attachment of molecules to a surface instead of complete assimilation in the bulk phase. In other words, absorption is a bulk phenomenon, whereas adsorption is a surface phenomenon.
Hope this helps.