Hi @Bud,
In ArcGIS Pro a feature layer refers to any map layer representing feature data. The feature layer will have it's data source as things like geodatabases, shapefiles, feature services, etc. A feature layer's definition is the symbology, definition queries, visibility, along with it's data source.
In our portal environments (AGOL and Portal for ArcGIS) the term feature layer (sometimes 'web feature layer') was chosen to represent a similar concept on the web. It's a little bit bigger of an abstraction in what this means. I will do my best to clarify. A good start might be to read through this doc: https://doc.arcgis.com/en/arcgis-online/reference/feature-layers.htm
Hopefully what I am about to say doesn't contradict that link 😀.
There can be different data sources for feature layers such as feature collections, but typically feature layers have a data source which is a feature service. A feature service can have one or many (sub) layers and these flow through to the feature layer in the portal. When you add a feature layer to your map you will get all of the sublayers in that feature layer added and for convenience we will typically group that into a group layer. The sub layers can also be added individually, in which case they will not be grouped. When you add layers to Pro, the individual sub layers become 'Pro feature layers' (not an official term). These 'Pro feature layers' have the same behavior as any other feature layer, with the key difference being that thee data source for these layers is a feature service.
From the Pro side of things I tend to think of 'web feature layer' = feature service = workspace these will be automatically grouped into group layers if you add them, while 'web feature layer sub layer' = feature service sub layer = feature class these will not get grouped into group layers.
Hope this helps.
