Explanation:
You can say that a function take input and outputs something back. for example, let's y = X + 3. You may think this is a equation.Yeah it is but it is also a function. The original function notation is for example f(X) = X+3.But we set y = f(X) so it turn out to be y=X+3.Basically these two have the same meaning.X is a input in the function above.If you set X= 5,the input is now 5 and you will get a kinda output of 5+3 =8.so y= 8 or f(5) = 8.That's all.You can check out more on the internet.