Muting a key means that the key no longer contributes to the animation on its channel.
To mute keys
In the Graph Editor Mute appears under the current animation channel’s name in the Outliner and a Mute curve with keys appears in the view area.
Graph Editor: Muted Key (circled) and the ignored region of curve/channel (dotted square)
The Mute curve has the same number of keys and they are positioned the same as those on its animation channel.
Dope Sheet Editor: Muted Key (circled) and the ignored region of curve/channel (dotted square)
To unmute keys
Select Keys > Unmute Key. The key now contributes to the animation on its channel. Unmuting a key does not remove the Mute curve. To remove the Mute curve from the current animation channel, see Mute channels.