A container is a box that contains other box
Many box positions and sizes are calculated with respect to the edges of a rectangular box called a containing block. See also: Container block
In general, generated boxes act as containing blocks for descendant boxes; we say that a box “establishes” the containing block for its descendants. The phrase “a box's containing block” means “the containing block in which the box lives,” not the one it generates.
Each box is given a position with respect to its containing block, but it is not confined by this containing block; it may overflow.