Petr and Pilou have already shown some good samples, I just want to show this somewhat more exaggerated one.

Given these curves to sweep:

Having no maintain height gives this:

Having maintain height turned on gives this:

The way that sweep works, it transforms the profile curve along the rails. This involves rotating and moving the profile, but when the rails move away or towards one another it also involves scaling the profile.

"Maintain height" controls how the scaling works. With it off, the profiles are scaled uniformly in all directions, just like the Transform/Scale command. With it on, the profiles are scaled in a stretch-like fashion, only in the direction between the rails - this is like Transform/Scale1D.

So in the example above - with maintain height turned off, the circle is kept a circle (because of the uniform scale) as it goes through the sweep. With it turned on, the circle is stretched into an ellipse shape as the sweep progresses.

With maintain height off, the resulting surface will bulge out more when the rails move apart from each other. With it on, the surface will keep a constant height relative to the profile.

- Michael