What is the meaning

A permille or per mille (Latin, literally meaning 'for (every) thousand') is a tenth of a percent or one part per thousand. It is written with the sign ‰ (Unicode U+2030)., which looks like a percent sign (%) with an extra 0 at the end. It is a stylized form of the three zeros in the denominator.
1‰ = 10-3.
For instance, 5‰ = 5/1000 = 0.005 = 0.5%

[edit] Examples
Examples of common use are:

Blood alcohol content
Seawater salinity. Eg "the average salinity is 35‰".
Tunnel gradients
Birth and death rates.
When used to describe property taxation in the U.S., it is called the millage rate.

[edit] Related units

Percentage point
Percent (%) 1 part in 100.
Basis point (‱) 1 part in 10,000.
Parts per million (ppm).
Parts per billion (ppb).
Parts per trillion (ppt).