A support prevents some motions of a structure and therefore exerts reaction forces or reaction moments on it.

The reaction components included in a model depend on which motions the support prevents.

Common planar supports

A roller support prevents motion normal to its contact surface, so it contributes one reaction force.

A pin support prevents translation in two directions while allowing rotation, so it contributes two force components.

A fixed support prevents both translation and rotation, so it contributes two force components and a reaction moment.

roller:   one reaction
pin:      two reactions
fixed:    two reactions + moment

Finding reactions

Replace each support by its unknown reaction components on the free-body diagram, then apply static equilibrium:

$$\sum F_x=0,\qquad \sum F_y=0,\qquad \sum M=0.$$

Choosing a moment point carefully can eliminate unknown reactions from one equation.

Why reactions matter

Support reactions are the external forces that make the structure balance the applied loads. They must usually be determined before internal member forces, shear forces or bending moments can be found.