IntelliJ / Android Studios: hide the project pane
That is the Escape (esc / ⎋) key.

You can find more explanations for Mac Keyboard Symbols here.

The click with option/alt/⌥ function exists to hide the entire sidebar. The sidebar can contain multiple panes, such as the Favorites pane. Without holding option/alt/⌥, it will hide only the Project pane and expand any other pane in the sidebar (if none, it will hide the sidebar).