What does 25 jewels in a watch mean?

There are many categories of jewel movement, for example, 17 jewels, 21 jewels, or 25 jewels. The actual meaning of these numbers is just a representation on the number of rubies used within the mens watches

Tiny synthetic gem crystals are set between many of the gears, in order to reduce friction. These gems resist temperature better than metal, and hold lubricant much longer.

Originally, the natural rubies were used before a major switch to industrial-grade or synthetic rubies for cost efficiency reason. There are two important points about rubies that make them suitable to be used as bearings.

Firstly they are extremely hard (it can be as hard as diamonds) to withstand high frequency of abrasion and wearing. Secondly, its smooth surface is perfect for the various steel components in the watch to operate on with much reduced friction.
With the high load and high speed motions within watches, the two advantages of ruby clearly outweigh any possibility of using steel bushings.

You can actually hear the jewel movement working. The ticking noise of mechanical watches are actually made by two rubies banging into the steel teeth of the escape wheel. You should make sure to have your watch serviced every five years, as the toughness of the rubies can erode the steel if not properly looked after.