Sum of Product is the abbreviated form of SOP. Sum of product form is a form of expression in Boolean algebra in which different product terms of inputs are being summed together. This product is not arithmetical multiply but it is Boolean logical AND and the Sum is Boolean logical OR.
To understand better about SOP, we need to know about min term.
Min Term
Minterm means the term that is true for a minimum number of combination of inputs. That is true for only one combination of inputs.
Since AND gate also gives True only when all of its inputs are true so we can say min terms are AND of input combinations like in the table given below. 
3 inputs have 8 different combinations. Each combination has a min terms denoted by small m and its decimal combination number written in subscript. Each of these minterms will be only true for the specific input combination.