close-rectangle is an alias for `delete-whitespace-rectangle' in `rect.el'.

(close-rectangle START END &optional FILL)

Delete all whitespace following a specified column in each line.
The left edge of the rectangle specifies the position in each line
at which whitespace deletion should begin. On each line in the
rectangle, all continuous whitespace starting at that column is deleted.

When called from a program the rectangle's corners are START and END.
With a prefix (or a FILL) argument, also fill too short lines.