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In The Light Fantastic, after talking about the dimensions of the Pyramid of Tsort, it says

All in all, it was a lot of effort to go through just to sharpen a razor.

What's the joke here?

In the real world, there is or was a belief, brought on by the Egyptomania of the early 20th century, that pyramids had special powers—including, specifically, the power to sharpen or maintain the sharpness of razor blades.

Pyramid power refers to the belief that the ancient Egyptian pyramids and objects of similar shape can confer a variety of benefits. Among these assumed properties are the ability to preserve foods, sharpen or maintain the sharpness of razor blades, improve health, function "as a thought-form incubator", trigger sexual urges, and cause other effects. Such unverified theories regarding pyramids are collectively known as pyramidology.

Czechoslovakian Karel Drbal even patented a pyramid-shaped device specifically designed for razor blades. Among the specifications:

It is beneficial to leave a new blade in the pyramid one to two weeks before using it. It is essential to place it there immediately after the first shave, and not the old, dull one. But it is possible to use an old one, if it is properly resharpened. The blade placed using the method above is left unobstructed until the next shave. The west edge should always face west. It improves the sharpening effect.

This is a reference to the popular myth that placing a razor blade inside a pyramid shape somehow confers magical powers on it, keeping it sharp, something that that inspired multiple patents in the 1950s.

Interestingly, this does actually work in the world of the Discworld, but not for the reasons you might think. Pyramids cause a slowing (and in extreme cases reversal) of time.

By the way, contrary to popular opinion pyramids don’t sharpen razor
blades. They just take them back to when they weren’t blunt. It’s
probably because of quantum.

_{^{Pyramids by Terry Pratchett}}

This is the legend of 'Pyramid Power'

From Wikipedia:

Pyramid power refers to the belief that the ancient Egyptian pyramids and objects of similar shape can confer a variety of benefits.

Among these assumed properties are the ability to preserve foods, sharpen or maintain the sharpness of razor blades, improve health, function "as a thought-form incubator", trigger sexual urges, and cause other effects.

Such unverified theories regarding pyramids are collectively known as pyramidology.

There is no scientific evidence that pyramid power exists.

The best resource to find the explanation of jokes in Terry Pratchett books is the trusty old Annotated Prattchet File on L-Space Web. If you look up the annotations for The Light Fantastic in it, and search for "razor", you will find a short explanation there.

• [p. 35] "He read that its height plus its length divided by half its width equalled exactly 1.67563..."

A parody of the typical numerical pseudo-science tossed about regarding the Great Pyramid and the 'cosmic truths' (such as the distance from the Earth to the Sun) that the Egyptians supposedly incorporated into its measurements.

The remark about sharpening razor blades at the end of the paragraph is similarly a reference to the pseudo-scientific 'fact' that (small models of) pyramids are supposed to have, among many other powers, the ability to sharpen razor blades that are left underneath the pyramids overnight.