What is 'Treemap' in Google Analytics ?

A treemap is a visualization that uses nested blocks to represent a hierarchy (tree) of your data. Each block represents a branch of the tree and contains smaller blocks that represent its "children" or sub-branches. At each level, the relative sizes and colours of the blocks represent the relative values of your selected metrics

Treemaps let you visually identify trends and trouble spots across your account with speed and insight that is hard to get when looking at numbers alone.