It refers to the angles that the tubes of the bicycle frame meet. Look at the frame of a road bike, and the frame of a urban/dirt jumper bike. THe top tube is almost horizontal on the road bike, yet the top tube is at a much steeper angle on the urban bike. Different angles effect your riding in certain instances. The longer and more spread out geometry of a road bike allows you to be much more efficient than the short seat tubed and steep top tubed urban bike. Yet the urban bike will handle a jump far better.
You want to find a frame that fits your interests for riding.
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