Frequency comb takes a measure of distance
In its simplest form, a Michelson interferometer is used with monochromatic light. However, this limits its effectiveness because, before a measurement is made, the length of the distance to be measured must be known to within one half of the wavelength (λ/2) of the light used – typically less than 500 nm. The problem is that the distance being measured can be expressed as an integer multiple of λ/2 plus a fraction of λ/2 – but this integer multiple cannot be determined from the interference data.
Physicists have found two ways round this problem. One is to use several lasers at different colours to gain more information about the system. The other is to use a light source with a range of wavelengths and then look for phase differences in the interfering light, which can be related to distance. These techniques have their own problems, not least that thousands of different lasers would ease a measurement but installing them all in a lab would be impractical.
A frequency comb has all of its wavelengths present in a single beam, though. Really cool application of the technology.
So a frequency comb works something like an old fashioned Vernier gauge which uses two scales with mutually prime divisions to provide much more precise readings than either scale alone. They used to use it to get the 3rd, or even the 4th, digit on micrometers.
It’s not that the comb improves the precision like a vernier. It removes the length ambiguity, so that the technique can be applied even if you don’t already know the length as precisely. A single wavelength gives you a result of (n 1/2)λ but you need to determine n.