How to find out what a button does

Whereany buttonicon or field of the editor

Steps

Point at a button and hold the cursor there. After about half a second a dark box with a hint appears.
Move the cursor to a neighboring button: its hint appears immediately, with no delay.

Good to know

If an action has a key, it is shown in the hint on the right as a small grey square: for Roof it is K, for Undo it is Ctrl and Z. For the tabs at the top, these are the numbers starting from 1.
Click, scrolling the mouse wheel or pressing a key hides the hint.
The i or ? icon next to a field name shows the explanation as soon as you point at it, and a click pins it. You can close it by clicking elsewhere, clicking again, or pressing Esc.
On a phone or tablet there are no hover hints.