The above image shows most of the buttons active in order to display them more clearly. However in a new project with no audio present or selected more of the buttons would be grayed indicating inactive.
Cut Ctrl + X or
Removes the selected audio data and/or labels and places these on the clipboard. By default, any audio or labels to right of the selection are shifted to the left.
Copy Ctrl + C or
Copies the selected audio data and/or labels to the clipboard without removing these from the project.
Paste Ctrl + V or
Paste either inserts the clipboard contents at the position of the selection cursor, or replaces a selected area with the clipboard contents.
Trim Audio Ctrl + T or
Trim Audio deletes all audio but the selection. If there are other separate clips in the same track these are not removed or shifted unless trimming the entire length of a clip or clips. Does not affect label tracks.
Silence Audio Ctrl + L or
Silence Audio replaces the currently selected audio with absolute silence. Does not affect label tracks.
Undo Ctrl + Z and
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 cannot 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.
Redo Ctrl + Y (on Mac Shift + Ctrl + Z or
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.
Zoom In Ctrl + 1 or
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 Ctrl + 3 or
Zooms out to a lower magnification level. You can zoom out so far as to fit 228 hours of audio on the screen.
Zoom to Selection Ctrl + E or
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 to Width Ctrl + F or
Zooms in or out so that the entire audio of the project fits in the available horizontal track area.
Zoom Toggle Shift + Z or
Zooms between two preset levels. These can be set using Tracks preferences.