It is function composition. If you have one function f(x), and another function g(x), then we can create a new function named g∘f (read as: "gg composed with ff") that is defined as(g∘f)(x)=g(f(x))(g∘f)(x)=g(f(x))For example, if f(x)=x+1f(x)=x+1, and g(x)=2x−1g(x)=2x−1, then(g∘f)(x)=g(f(x))=g(x+1)=2(x+1)−1=2x+1