Know Your Lines
The first line is the edge of the board. Playing unsupported stones on the first line is a typical novice's mistake. Moves on the first line are common in the endgame, and in life and death.
In Sensei's Library, stones on the second line are referred to as very low, as in "very low approach". Crawling along the second line to make territory is inefficient, therefore it is also called "the line of defeat". However, many large endgame moves are on the second line.
Stones on the third line are referred to as low, as in "low approach". Territorial moves are on the third line, therefore the third line is also called the "line of territory".
Stones on the fourth line are referred to as high, as in "high approach". In Japanese go terms the element taka (高) means high. Moves on the fourth line emphasise influence, therefore the fourth line is also called the "line of influence". Fourth line territory, if you manage to make it, is usually very efficient, therefore the fourth line is also called the "line of victory".
Stones on the fifth line are referred to as very high, as in "very high approach". Stones on the fifth line are usually played when other stones are already present. This is the line where stones extending a formation towards the center or capping plays are often played.