It is the feeling that the steering wheel is pulling one way then the other, as the front tires follow grooves in the road. Wider tires will tend to tramline more, althrough it does depend on the specific tire and tread somewhat.

Imagine streetcar tracks (tram lines) years ago, and what it would feel like from the driver's seat as you were crossing the tracks, at an angle close to parallel.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue