What does sketch function in AutoCAD mean?

You can use the SKETCH command to draw freehand sketches. Sketching is useful for creating irregular boundaries or for tracing with a digitizer

This is the explanation for the sketch command from the AutoCAD help file: Sketching is useful for creating irregular boundaries or for tracing with a digitizer. Specify the object type (line, polyline, or spline), increment, and tolerance before sketching.

I hope this helps.