Yes an air lock. Each manufacturer has their own procedure for filling---the BASIC strategy is to open the air relief valve on the heater hose and fill the coolant through the open radiator cap until it overflows through the plug. Then you button it up, install the radiator cap and warm up the engine. Then race it a couple of times, shut it down and let the engine cool (not cold, just around 120 degrees or so). Then refill your reservoir tank (don't open the rad cap). Repeat this a couple of times until the coolant level no longer drops in the reserve tank. If you hear any kind of gurgling or noise in the area of your heater box, you still have air in the system.
When you pour coolant in the radiator (we are talking about the radiator, not the reservoir) you have to pour about 2 quarts a minute, no faster.
so the general idea is a) fill up your coolant into the radiator until the overfill plug spills out, then b) button up and with repeated fillings through the RESERVOIR tank, keep doing it until the coolant no longer drops between cool downs.