The Quickfix List

Filling it

:vimgrep /TODO/ **/*.py
:grep -rn "pattern" .

Diagnostics, test runners and build tools populate the same list.

Walking it

CommandDoes
:copen / :ccloseshow / hide the list
:cnext / :cprevnext / previous entry
:cfirst / :clastjump to the ends

The list is a work queue: every entry is a file and a line you still have to deal with.