96 Jetta oil blow by? What dose this mean?

It means exactly what it says. Oil is blowing by the rings. It comes with wear. The mechanic should have been more up front with you on is it bad.
The metal parts (piston rings and cylinder walls are in near contact with each other seperated by a skin of oil) This oil slows down the wearing away of the metal parts. But they do wear. As they wear, there is more of a gap between the ring and the wall and compression pressure sneaks past down into the crankcase where the oil is held. Thru the various hoses to releave the pressure in the crankcase this compression would be pumped out into the air cleaner as a visible oily mist Blowby. As the engine gains more miles oil sneaks by the rings upward into the combustion chamber and is burnt with the gas giving you the blue smoke. Engine is worn quite a bit at this stage, meaning an engine job should be happening. With the older cars 70's with the oils that were available, this stuff would be seen around 100,000 miles. With the newer oils, and regualar oil changes the mileage should have increased between engine jobs.