One of original great improvements of VIM over Classic Vi was the ability to reuse/edit/correct commands laboriously typed in, you had only simple line edit commands available to do this, normally perfectly sufficient of course. However you also open a command line window to gain full vim command line editing commands:-
Note these are normal-mode commands!
Creates a small window of your most recent commands
Creates a small window of your most recent searches
hitting "return" will execute whatever line you are editing
to close/leave the command window without executing the current line use :quit