Constraints can be added to sketch elements to create geometric controls and relationships. By defining constraints you can add information to your sketches that control their behavior when they are modified. For example, you can add a Horizontal/Vertical constraint to a line to ensure it remains aligned with the axes of the sketch plane.
When working in a sketch, valid constraints for selected elements are highlighted in the constraints menu. You can also choose to have constraints created automatically for you while sketching by adjusting options in Constraint Settings.