A register can contain a rectangle instead of linear text. See Rectangles, for basic information on how to specify a rectangle in the buffer.
C-x r r r
Copy the region-rectangle into register
copy-rectangle-to-register). With prefix argument, delete it as well.
C-x r i r
Insert the rectangle stored in register
r (if it contains a rectangle) (
C-x r i r (
insert-register) command, previously documented in Text Registers, inserts a rectangle rather than a text string, if the register contains a rectangle.