A constraint is a rule applied to a sketch that locks in a size, angle, position, or relationship between objects. In parametric CAD, constraints replace guesswork with defined geometry: a line can be fixed to a length, two edges can be made parallel, points can be coincident, or a shape can be centered on an axis.
In FreeCAD, constraints matter because they keep models controlled and editable. Instead of redrawing a part when a dimension changes, you update a constraint and the sketch regenerates from the new rule set. This is the core of parametric modeling, but it also requires care: too few constraints leave geometry underdefined, while conflicting constraints can make a sketch impossible to solve. For beginners, learning constraints is often the step that turns a loose drawing into a reliable 3D design.