Granite is dense, but it isn't completely nonporous. A penetrating sealer fills the tiny pores near the surface so oil, wine, and coffee bead up instead of soaking in. Over time the sealer wears down, especially near the sink and cooktop.
To check, pour a small puddle of water on the counter and wait fifteen minutes. If the water still beads and the stone underneath looks unchanged, your seal is fine. If the stone darkens where the water sat, it's time to reseal.
Resealing is simple. Clean the counter with a pH-neutral stone cleaner and let it dry completely. Apply a granite sealer evenly, let it sit for the time on the label, usually a few minutes, then wipe off the excess with a clean cloth so it doesn't haze. Let it cure before heavy use.
For daily cleaning, skip vinegar, bleach, and lemon-based cleaners, which break down sealer faster. Warm water with a drop of dish soap, or a cleaner made for stone, keeps your counters protected for longer.