The aerator is the small screened fitting on the end of your faucet spout. It mixes air into the stream to cut splashing and save water, and its fine screen catches debris. In hard-water areas like Arizona, calcium and magnesium scale collects on that screen and inside the housing, slowly choking the flow. If one faucet has weak or sputtering pressure while the rest of the house is fine, a clogged aerator is the most common and easiest cause to rule out first.
To clean it, first close the drain so you don't lose parts, then unscrew the aerator from the spout by hand, turning counterclockwise as you face up at it. Most thread off with finger pressure. If not, wrap the housing in a cloth and use pliers gently so you don't scratch the finish. Note the order of the parts as you take them apart, the housing, screen, and rubber washer, since they've got to go back the same way. Lay them out in sequence on a towel.
Soak the disassembled parts in a bowl of white vinegar for at least a few hours, or overnight for heavy scale, to dissolve the mineral deposits. After soaking, scrub the screen and housing with an old toothbrush, and use a toothpick or pin to clear any individual holes still blocked. Rinse everything well, put it back together in the original order, thread the aerator back onto the spout hand-tight, and run the water. Flow should come back to normal if scale was the problem.
Sometimes cleaning isn't worth it. If the aerator is corroded, cross-threaded, cracked, or just won't come clean, replacing it is the better move, and aerators are cheap. Bring the old one to the hardware store to match the size and thread type, since faucets use male or female threads in a few standard diameters, or take the brand and model. Adapters exist for nonstandard faucets. Wrapping the threads with a turn of plumber's tape before installing helps stop leaks and future seizing.
Because Arizona water is so hard, expect to do this every few months on heavily used kitchen and bathroom faucets. A few habits help. Clean aerators on a schedule rather than waiting for flow to drop, and think about a water softener, which dramatically cuts how fast scale forms throughout the house, aerators included. If you clean an aerator and pressure is still low at that fixture, the restriction is likely further upstream in the supply line or valve, which is worth having checked.