Undo

This will undo the last editing operation you performed to your project. Audacity currently supports full unlimited undo - meaning you can undo every editing operation back to the last time the document was saved.

Redo

This will redo any editing operations that were just undone. After you perform a new editing operation, including simply resizing a track, you can no longer redo the operations that were undone.

Cut

Removes the selected audio data and places it on the clipboard. Only one "thing" can be on the clipboard at a time, but it may contain multiple tracks.

Copy

Copies the selected audio data to the clipboard without removing it from the project.

Paste

Inserts whatever is on the clipboard at the position of the selection cursor in the project, replacing whatever audio data is currently selected, if any.

Delete

Removes the audio data that is currently selected without copying it to the clipboard.

Silence

Erases the audio data currently selected, replacing it with silence instead of removing it completely.

Insert Silence...

Prompts for a dialog indicating the number of seconds of silence to insert, then inserts that amount of silence at the current selection cursor position in the project, replacing whatever audio data is currently selected, if any.