Edit Toolbar

From Audacity Manual

These tools perform the same functions as those accessible through the Edit menu, View menu and keyboard shortcuts.

Editing

 Cut: Removes the selected audio data and places it on the clipboard.

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

 Paste: Either inserts the clipboard contents at the position of the selection cursor, or replaces a selected area with the clipboard contents.

 Trim: Deletes everything but the selection.

 Silence: Replaces the currently selected audio with absolute silence.

Undo, Redo

 Undo: Reverts the last editing operation. Audacity supports unlimited "stepwise" undo. This means you can undo every editing operation back to the last time the project was opened, but you can't undo one particular edit without also undoing any changes made after that edit. You can review your editing history and jump straight back to any particular edit point at View > History.

 Redo: Restores the previous editing operation that was just undone. Note: If you undo an operation then perform any new one that appears in the Undo History, you can no longer redo that undone operation.

Zooming

 Zoom In: Zooms in to a higher magnification level. You can continue to zoom in until you reach the level of displaying individual audio samples.

 Zoom Out: Zooms out to a lower magnification level. You can zoom out so far as to fit 228 hours of audio on the screen.

 Fit Selection: Zooms a selection region in or out so that it fits in the available horizontal window area. The button is therefore grayed out unless a selection region is made.

 Fit Project: Zooms in or out so that the entire audio of the project fits in the available horizontal window area.