In the multi-cursor editing, the same text can be input at multiple desired points at once.
You can add your insertion points (cursors / selections) by the following ways:
To add an insertion point at the desired location, click or drag the point while pressing Command.
To add insertion points at the same column in multiple continuous lines, drag the area while pressing Option (rectangular selection).
To add an insertion point just above or below the current insertion point, press Control-Shift-↑ or Control-Shift-↓.
To remove one of the multiple insertion points:
Command-click the insertion point to remove.
To end the multi-cursor editing mode:
Click or drag somewhere in the editor without modifier keys by setting a single insertion point at the location.
Press Esc key and all insertion points except the top one are discarded.
The multi-curor editing currently doesn't support the text input that requires user selection after input, such as Japanese. In such cases, multi-cursor editing finishes automatically when typed and text is input only at a single point.