A TXV (thermostatic expansion valve) is the metering device that controls how much refrigerant flows into the evaporator coil. It sits between the liquid line and the evaporator inlet, and it modulates flow based on superheat — the temperature of the refrigerant vapor leaving the evaporator.
TXVs improve efficiency over older fixed-orifice metering devices (sometimes called "pistons") because they adjust to varying load conditions. On a hot afternoon they let more refrigerant through; on a mild evening they restrict flow.
TXVs occasionally fail — usually stuck open (overfeeding the coil, causing flooding back to the compressor) or stuck closed (starving the coil, causing low cooling and ice-up). Failure diagnosis usually requires superheat / subcooling measurements, not just gauge pressures.
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