A truss is a structure made up of triangles. There are three main reasons that triangles are used to form trusses: their unique geometric properties, their method of transferring loads and their spatial openness.
A triangle has three sides and three angles, and each angle is held solidly in place by the side opposite it. This means that a triangle's angles are fixed, and that if pressure is placed anywhere on a triangle, its angles, unlike those of other shapes, will not change.
When a force is applied to a triangle, the resulting pressure is directed sideways rather than down. This means that sides of the triangle are in either compression or tension, and that there is thus no bending movement.
Since the centre of a triangle does not contribute to its geometric rigidity or structural integrity, the centre of a triangle can remain open. As one of the goals in erecting a truss is minimising its weight, triangles are an appropriate shape to use.