When we speak of the third line, this is what we mean:
On a 19x19 goban, it is said that the third line is the line of territory. This means that a fair exchange can result into stones on the third line facing the side, opposing stones on the fourth line building influence.
It is also said that "the third line is the line of completion". This means that playing on the third line is usually an attempt to complete a secure shape, rather than develop a big framework.
When speaking of extensions or approach moves, the third line line will be referred to as low, whereas the fourth line is called high. The third line was used extensively in Edo period go. Gradually the fourth line was used more for extensions.