A row that gathers several buttons — sometimes a small input like a spinbutton too — so Tab passes over the whole toolbar in one stop and arrow keys move between items inside it. This "roving tabindex" technique means ten or twenty buttons never cost ten or twenty Tab presses. The container is role="toolbar", only one item holds tabindex="0" at a time (the rest are -1), and that tabindex moves to the next item on every arrow-key press.
The difference from a menubar, covered there, is what happens on press — a toolbar's buttons run immediately (Save) or toggle a state (a starred favorite), and an item that opens a submenu is the rare exception mixed into a toolbar, not the rule. The rich text toolbar (see that entry) is a specific case of this pattern, with one extra requirement: its buttons must keep recomputing their pressed state to match the current selection's formatting.
Buttons aren't the only members — a small spinbutton like a zoom indicator can join the same roving-tabindex group. The core idea is that a toolbar isn't "a bunch of buttons" so much as "a group of controls sharing one Tab stop."