First line
The first line is the entire outside line defining the playing area on the go board.
A play on the first line is rarely good by itself. Such plays are made, usually, in combination with existing stones on the second line. Playing unsupported stones on the first line is a typical novice's mistake.
Plays on the first line become common during endgame, and in life and death
Playing rather than a can be a good choice in endgame terms, but this is a play with hidden subtleties.
The cutting point marked with a circle could make some difference to the game, though. It has left some ko threats for White.
unkx80: I guess we need a better example. But all these have the notion of temperature.
In the typical endgame setting, and
would be sente. Compared to the next diagram, a White cut at
would be large, so
has to defend.