Offset creates parallel sketches to a sketch. It doesn't just apply to single edges (though it can), but to closed sketches and faces as well. Depending on the desired size of your final shape (compared to the original geometry), you can create positive and negative offsets.
You can access the Offset tool
- in the Sketch menu, by simply starting or selecting a sketch
- or in Tools > Offset.
After tapping on the icon, you will be brought to the Offset tool, where you will have the option to select if you want to offset a single edge or a sketch filling.
- single edge on/off
After selecting this, tap on any edge and start dragging it in (negative) or out (positive). If you can't hit the mark with dragging, just enter the dimension manually on the top left Preferences Panel.
In this case, I have to create a hole on the top side of this shelf. Before a subtraction, I have to draw a sketch, that is parallel to the skeleton of the shelf. This is when the Offset tool comes handy.
Tutorial (turn on the subtitle for detailed explanations)