Volume measures the space inside a three dimensional object, like a box (also called a "rectangular prism").
To calculate the volume of a box, you need three of its dimensions.
First, you must know the length of its base.
Second, the width of its base.
Third, the height of one of its sides.
Those are the three dimensions that make an object "3-d" - length, width, and height.
Just as in Area, you must multiply the dimensions to find the volume. The result will be in cubic units.
For this box, the dimensions are : Length = 5, Width = 8, Height = 6.
Volume = 5 x 8 x 6 = 240 cubic units.