A feature set is a data structure that contains a collection of features. It is an in-memory data structure that is used by APIs such as [ArcGIS Maps SDK for JavaScript], ArcGIS Arcade, and ArcGIS API for Python. (https://developers.arcgis.com/javascript/latest/).
A feature set can be created by querying or filtering data from a layer or dataset in ArcGIS, and it can contain multiple features with associated attributes. The features can contain points, polylines, or polygons.