Integration is a reverse process of differentiation. Suppose you consider a function such as f(x) = 2x, then F(x) = x2 that's very simple because F'(x) = 2x. Hence the function F(X) is called the base function of f(x). So now we proceed even telling this as F(x) = x2+ C. Now if we want to find the base function then we use ∫ f(x) dx which gives x2+C. The symbol ∫ is called the indefinite integral.