Air Conditioner Is Freezing Up

The smaller copper line that runs into the side of my ac unit inside is freezing up. I know to turn the air off and the fan to constant, but I do not know what's causing this. Is it low on coolant?

The smaller copper line that runs into the side of my ac unit inside is freezing up. I know to turn the air off and the fan to constant, but I do not know what's causing this. Is it low on coolant?