The rule of thirds is a safety measure commonly used when diving overhead environments such as caves, caverns, and wrecks.
The typical scuba tank has 3000psi or 210 bar. With the rule of thirds, you use 1/3 in, 1/3 out, and 1/3 as reserve. Therefore, when you reach 2000psi or 140 bar you turn the dive and begin heading back the way you came. In theory you will use the same amount of gas while traveling that distance and will maintain the final third in the event something doesn't go as planned.
It is a much more conservative diving style due not having direct access to the surface.