A lane is a path traveled by a glider, or less commonly a spaceship such as a loafer. The lane is centered on the line of symmetry (if any) of the spaceship in question. If a lane is clear, then the spaceship can travel along it without colliding or interfering with any other objects.
Diagonal lanes are often numbered consecutively, in half-diagonals (hd). Occasionally diagonal lane measurements are given in quarter-diagonals (qd), in part because diagonally symmetric spaceships have a line of symmetry 1qd away from the lines available for gliders. It's also convenient that moving a glider forward by 100qd (for example) has the same effect as evolving the same glider for 100 ticks.