A sketch is an element composed of geometric and constrained elements.

Creation Stages:

In a Drawing document, click on the icon or select the Sketch > Sketch... command from the drop-down menu.

Each activation of the command creates a new sketch which is set as current.

Additional Information:

The current sketch is shown with bold font in the sketches list in the entities tree.

You can change the current sketch through the contextual command "Make current"  on a sketch in the entities tree.

A sketch can absorb other sketches through the contextual command "Absorb".