Ben

Blowby is combustion gases that gets forced by the piston rings into the crankcase. Then gets mixed into the combustion chambers.

It's evident by blue smoke coming out of the exhaust pipe when you start the car up after it's been sitting for a while. As the problem worsens you will start to notice the blue smoke whenever you come to a stop and accelerate again. When it's real bad the smoke will be pretty much constant all the time.

It's oil that can be leaking past worn piston rings/cylinders/valve guides/valve stem seals. There are tests that can be performed to locate the problem. If left unchecked it will eventually do damage to the engine.

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Mike