Like when they say 700c or something. What is that? Is it centimeters?

It means...nothing.

I wish I could say that it means 700 centimeters, but that'd be a wheel almost 23' in diameter. It doesn't refer to circumference either as it's a nominal tire diameter. If compared to the tire width, it could very well stand for millimeters since a 23c tire can sometimes be 23mm in width, but it depends on what width of rim it's mounted to. To confuse further, the actual diameter is rarely 700mm exactly as tire width changes the diameter.

In summary, think of it as a women's dress size: It has no correlation to any accurate measurements whatsoever...you just need to remember your size.

It means nothing. In the old days there were several rim sizes that would all use tires of the same outside diameter; 700a, 700b, 700c. Currently nearly all road bikes (and 29er mountain bikes) use the 700c wheels with different widths of tires. You still see two different 650 rims still in use; 650b and 650c.

The 700 refered to the outside diameter of the original tire that was used on that size of rim. The bead diameter of the 700c rim is actually 622mm.

Thanks. And what does "bead" mean?

From Sheldon Brown - perhaps this will clarify---

"French sizes:

In the French system, the first number is the nominal diameter in mm, followed by a letter code for the width: "A" is narrow, "D" is wide. The letter codes no longer correspond to the tire width, since narrow tires are often made for rim sizes that originally took wide tires; for example, 700 C was originally a wide size, but now is available in very narrow widths, with actual diameters as small as 660 mm.

French Size ISO Applications
700 A 642 mm Obsolete
700 B 635 mm Rod-brake roadsters.
700 C 622 mm Road bikes, hybrids, "29 inch" MTBs.
(28 x 1 1/2 F.13 Canada)
700 D 587 mm Oddball size formerly used on some GT models.
650 A 590 mm French version of 26 x 1 3/8; Italian high-performance bikes for smaller riders
650 B 584 mm French utility bikes, tandems, and loaded-touring bikes; some older Raleigh and Schwinn mountain bikes
650 C 571 mm Triathlon, time trial, high performance road bikes for smaller riders
600 A 540 mm European Juvenile road bikes, most wheelchairs
550 A 490 mm European Juvenile road bikes
500 A 440 mm European Juvenile, folding
450 A 390 mm European Juvenile
400 A 340 mm European Juvenile "

https://sheldonbrown.com/gloss_ba-n.html#bead

Sheldon Brown is the late guru of cycling. https://sheldonbrown.com/