Godot will offer to add a RESET animation track when you decide to key a parameter. This RESET animation is just a special string that is used for setting a "default pose". The editor will also reference this animation when opening the scene again in the editor.

Animations generally change the mutable state of the scene tree, which is why the RESET animation can be useful. However, it will not reset variables that you've changed with animations that you have deleted.

Source: docs.godotengine.org