A small label that appears on hover or focus to name or explain the element under the pointer, taking no permanent room in the layout.
Give the bare icon a tooltip to name it on hover, but give the control a real accessible name too; the tooltip can’t be the only label.
A tooltip is a footnote, not a hiding place. Anything needed to operate a control has to be readable without it, because hover does not exist on touch and the tip is invisible to most assistive tech. Use it to confirm or enrich, never to hold the only copy of a label. And make it wait a beat before showing, or a toolbar turns into a minefield of popups.