A colinear constraint forces two lines to follow the same infinite line.
In the example below, a colinear constraint has been applied to the two lines on the left. The result is the block definition on the right, where the two lines are aligned.
Manipulate a Colinear Constraint
When you manipulate a block with a colinear constraint applied, the specified lines will always be aligned with each other.
In the example below, the constrained lines on the top have been rotated to produce the block on the bottom. The two lines can move, change angle, or change shape, but will always be constrained to align.