What is a frustum ? How to calculate its volume ?

In secondary school maths, frustum is the lower part of a pyramid.
It can be found by first finding the total volume of a pyramid, then minus the upper part of the pyramid.
Similar triangle's technique will be found useful in your calculations.
You may refer secondary three textbooks to see examples.

In higher maths, a frustum is the portion of a solid – normally a cone or pyramid – which lies between two parallel planes cutting the solid. Degenerate cases are obtained for finite solids by cutting with a single plane only.

You may refer to my reference as stated belows: