An oil catch can or tank is a device that is fitted into the cam/crank case ventilation system on a car. It's purpose is to reduce the amount of oil and oil vapors re-circulated into the intake of the engine.
PCV or positive crankcase ventilation is necessary to ensure there isn't a build up of pressure in the crankcase. Without releasing the pressure in the crankcase, it could cause it to crack under pressure. So, the PCV releases that pressure along with the oil and oil vapors back into the intake tract of the engine where it then coats the intake, intercooler if your car is turbocharged or supercharged, intake manifold and then the head where it coats the valves to be re-burned. This coating will coat cooling fins in the intercooler and reduce efficiency, interfere or damage sensors, and coat and solidify on the valves. A proper baffled catch can will separate the oil out of the air and send the clean air back into the intake protecting those parts from premature failure and reduced efficiency.