Those markings are focus distances in meters, and the switch is a focus limiter.

It limits the range the autofocus system will search through, which can make AF faster and reduce focus hunting.

In general:

• Use the longer-distance/full-range setting for normal shooting, such as portraits or subjects farther away.
• Use the close-range setting when shooting macro or other nearby subjects.

Why it helps: macro lenses have a very large focusing range, so autofocus can hunt a lot if it searches from very close focus all the way to distant subjects. The limiter tells the lens to ignore part of that range.

A good rule of thumb is to use the highest distance limit unless you need to focus closer.

Also, for true macro work, manual focus is often preferred because depth of field is very shallow and autofocus may not choose the exact point you want sharp.