Constraints in Autodesk Fusion 2D Sketch mode are used to limit the degrees of freedom of lines and shapes. They also reduce the amount of dimensions that need to be placed to fully constrain a drawing. For instance, if two lines share a Perpendicular constraint, there is no need to add a 90 degree dimension between them, that would in fact even over-dimension the drawing.
The available constraints are:
Horizontal/Vertical
Coincident
Tangent
Equal
Parallel
Perpendicular
Fix/UnFix
Midpoint
Concentric
Collinear
Symmetry
Curvature
Episode 15 of a step by step tutorial series of Autodesk Fusion (formerly called Fusion 360).