A trap primer is an important plumbing device that adds water to a trap. With no water in a trap, dangerous / explosive sewer gases (methane) can enter your home.
Evaporation of the water in a plumbing trap can occur if not replenished. A solution to this problem is the installation of a trap primer. They may be required, for example, if your furnace condensate drains into a trap in the winter but there’s no air conditioner also draining into that same trap in the summer thereby allowing it to dry out. Perhaps the trap for your basement floor drain never gets water added to it or there’s a trap located in a building where explosion hazards are high. In these cases, a trap primer is a must. If you smell sewer gas, it may be an indicator that there is not water in the plumbing trap.
Some laundry tub faucets have built in trap primers that you connect a small plastic hose to for the water supply, while other trap primers are connected directly to the water supply line. Either way, they provide the essential water needed to prevent the trap from drying out by having water slowly drip from them.
If you are buying a home (or own a home) and are unsure whether you require a trap primer, consult a professional.