This is an optional thin slot table insert for the AC216TS table saw. It is used with the saw blade set at vertical only and is designed to prevent material going down past the blade when cutting thin strips.
It will also help prevent tearout of the grain on crosscuts. The slot is just wide enough for the blade kerf. When fitting for the first time it may be worth setting the blade at its lowest position, fitting the insert, switching the saw on then raising the blade very slowly to its full height. Being alloy, if it catches the blade slightly there will be no damage to the blade.