Some geoprocessing tools (the Add Join tool, for example) only accept in-memory layers as inputs. You can create these layers with the Make Feature Layer tool.

To make a layer for the vegtype feature class, expand the GP_Tutorial connection, then click the plus sign next to the Habitat_Analysis.gdb to view its contents. In the ArcToolbox window, expand the Data Management Tools toolbox, then expand the Layers and Table Views toolset. Right-click Make Feature Layer and click Open or double-click the tool. Drag the feature class vegtype from within the Habitat_Analysis geodatabase into the Input Features text box. Type vegtypelayer for the name of the in-memory layer to create, and click OK.

An in-memory layer is created. You cannot see this layer in the ArcCatalog tree, but it can be used as input to geoprocessing tools that support it.