Tension vs Compression
The two basic types of physical stress are compressive stress and tensile stress.
Consider a length of material, be it a stone slab, steel bar, or piece of lumber. If one were to hold each end of the material and squeeze, one would exert compressive force.
If, however, one were to pull outward (as though one were trying to stretch the material), one would exert tensile force.
The compressive and tensile strength of building materials is of primary importance to architects. Consider a simple structure that consists of two posts supporting one beam. Atop the beam is a very heavy weight (e.g. a stone roof) represented by the thick arrows.
In order for this structure to be sound, the two posts must have good compressive strength, while the beam must have good tensile strength. Although the beam isn't experiencing pure tensile stress, its tensile strength is the limiting factor in whether it buckles under the weight of the structure above.
Prior to the modern age, when it became possible to mass produce steel economically, architects were limited to materials lacking in tensile strength (e.g. stone, brick, concrete). This compelled the development of arched construction, which converts most of a structure's tensile stress into compressive stress.