find walks a directory tree and tests each entry. A name pattern and a regular-file test narrow the selection before an action runs. Quote the pattern so the shell doesn't expand it against only the current folder.
The placeholder stands for each matching path, and the escaped semicolon ends the action. Start by inspecting matches when working outside an exercise. Here the retained text record makes an overly broad deletion easy to detect. The tests -type f and -name '*.tmp' both need to match before the action runs. The star is a filename wildcard, not a regular expression. In -exec rm {} ;, find substitutes the selected path for {}. The backslash protects the semicolon from the shell so find receives the action's terminator.