The term comes from old Arabic chess, where games began from a set position. Today a tabiya is the well-known tabletop a line funnels into — for example the main-line Italian after the pieces are developed — recognized instantly by anyone who studies that opening. Reaching it quickly conserves clock time and mental energy for the real battle ahead.
Thinking in terms of tabiyas is an efficient way to study openings: instead of memorizing endless move orders, you learn the handful of key positions a system produces and the typical plans, pawn breaks, and piece placements that flow from each one.