I'm reading a drum notation book and it talks about "kick lines"?
Is there a precise definition somewhere? (searching doesn't help since it thinks I'm talking about the kick drum)
JBarry's answer seems to define what a kick is, your comments seem to be asking what to do with it.
Percussion parts, especially drumset parts, are very rarely notated beyond general sketches, but it is expected that the player will blend with the ensemble and play tasteful, appropriate patterns accentuating what is happening throughout the ensemble.
That is the percussionist's responsibility, whether he doesn't get music at all, gets a blank page, or gets every stroke written out.
I'm mostly a classical guy, and this notation is very common in classical notation (though it isn't called a kick).
Traditionally, this notation was used as cues to prepare the drummer who had just played the same pattern 128 times that something new was coming. It's a signal to help keep count, track themes, and generally "have a clue."
In more modern times with the score left mostly blank, the notation also helps the percussionist figure out what type of thing to play in that location. The "kick" is still a cue but it also helps to guide your performance, hinting at what part should be accented. This is especially useful when music is performed with minimal rehearsal.
To answer your question, notation like this isn't prescriptive (defining what do do), but descriptive. There's nothing you "have to" do, and different performers will use the kick in different ways. The purpose is to make the artistic percussionist's job easier.
Kick lines (or just "kicks") indicate a rhythm being played by another instrument in an ensemble. They will usually be shown just above the staff, along with the name of the instrument that is playing that rhythm. They may be used as cues or just a guide to interpreting the drummer's part.
Reference: Norman Weinberg, "Guide to standardized drum notation", 1994 - a summary is here: http://www.propercussion.org/filer/notation.pdf