What is an area under curve?

It is still an area. In simple integral calculus, you integrate the function between limits on the x-axis. This will give the area under the curve.

e.g.

Taking y = X^2 between the limits of (x = 0, & x = 2) .

Integrated it is it is [x^3/3] between 2 & o)

Hence

[2^3/3] - [0^3/3] =

8/3 - 0 =

8/3 = 2 2/3 = 2.6666..... units^2

Hope that helps!!!!