Constraints are rules that you can apply on sketches so your drawings are accurately defined and resistant to transformations. This can make your rough sketch more precise and easy to create.
When you select a sketch or the Sketch in the main menu, the constraints will appear on your screen. Once you select lines, points, arcs, circles, edges of bodies, etc., then you can apply constraints by selecting the appropriate constraint tool. If you select a dimension label, whether it be an angle or length, that dimension will automatically be locked.
To delete a constraint, select the sketch you want to modify. Select the constraint icon on the sketch and then X.
These are the constraints you can use:
- Auto-Constraint
Auto-constraints are built-in mechanics to make your sketching experience faster and more precise. Instead of manually picking and applying constraints to your sketch, you can turn this on so the app can automatically recognize your intent and take care of constraints for you. For example, if your line is almost parallel to the X-axis, then auto-constraint fixes your line to be horizontal. - Show Constraints
Turn this on so you can see constraint icons on the sketch. - Parallel
Select two or more lines to be parallel to each other.
- Perpendicular
Select two or more lines with a right angle between them. The lines don’t need to share a common point or intersect with each other.
- Tangent
Make circles, lines, or a combination of the two tangent to each other.
- Concentric
Make two or more circles or arcs share their center points.
- Horizontal/Vertical
Make your lines parallel either to the horizontal or vertical axes of the sketch plane, whichever takes fewer adjustments.
- Equal
Make your lines equal in length and your circles or arcs the same size (radius).
- Symmetry
You can make your lines, circles, arcs, or points symmetrical to each other in relation to a selected axis.
For lines, the vectors will be mirrored to a given axis. For circles, the radii will remain the same and the center point will be on the same imaginary line, perpendicular to the axis of symmetry. - Disconnect
You can delete the connection between multiple connected points by selecting the common point and then
. When you disconnect points like in this GIF, the end points of the two sides are no longer connected so you can move the points separately and freely. You can connect the points again by dragging one point over the other. 
- Lock
You can lock all points on your selection, whether it be lines or circles. You can also lock one point by selecting the point and then
. 
Watch this tutorial to learn how to use constraint tools: