Constraints limit changes and define the shape of a sketch. For example, if a line is horizontally constrained, dragging an endpoint changes the length of the line or moves it vertically. However, the drag does not affect its slope.
As you sketch, constraints are automatically applied to the various sketch elements. For example, if the horizontal or vertical symbol displays when you create a line, then the associated constraint is applied. Depending on how accurately you sketch, one or more constraints could be required to stabilize the sketch shape or position.
Although you can use unconstrained sketches, fully constrained sketches result in more predictable updates.