Legacy Animation system

Unity’s Legacy animation system (Legacy) has a limited feature setA feature set is a collection of related packages that you can use to achieve specific results in the Unity Editor. You can manage feature sets directly in Unity’s Package Manager. More info
See in Glossary that predates Mecanim. Legacy is still available because it is easier to use and provides better performance for simpler animations.

Legacy is also available for backwards compatibility with old Unity projects. Legacy uses the Animation component and the special Legacy import option when importing legacy animation.

Working with Legacy Animations

To import a Legacy Animation, select Legacy as the Animation Type in the Rig tab of the Import Settings window:

Make sure you enable the Import Animation option on the Animation tab.