Imaging you have a histogram. There are multiple points along the X axis that have their own height. We want to find the largest rectangle on this graph.
For example, if we had a graph with three bars with hights 4, 3, 5 and 6 ([4,5,6]), the areas of these bars would be 4, 3, 5 and 6, but there is a 3x4 rectangle that overlaps all of these, thus the max area is 12.
There are multiple elements in an array. Each element represents a new bar along the X axis and the height is replresented by the value of the element.