means The clickable strips at the top of a browser window (or a notebook, file, or tab-key indent) that let you switch between separate pages or sections.

from From "tab," a short tag or projecting flap, a word of obscure origin that surfaced in English around the 17th–18th centuries — possibly a clipped, dialectal cousin of "tag" and "tap." It long meant a small protruding strip (the tab on a folder, the tab key that jumps a fixed distance), and digital interfaces borrowed that filing-cabinet image directly: each page gets its own little labeled flap to grab.