To Consider as Minterm, we have to take input combinations for which the function (Digital Circuit) gives us '1' as output.
if you consider a'b'c' as minterm, For input combination (0,0,0) it will give output as '1' (1.1.1 = 1) for which our function produce '1' as shown in the above truth table.
Consider only terms which produce output 1 or 0 but not both.
if you consider terms with output '1' they are written as sum of the product terms i so it is called SOP expression.
otherwise they are called as POS terms.
Hope your doubt is clear.