How an element or compound will easily gain or lose electrons when interacting with other elements. Reactivity can also be a measure of how violent a reaction is giving our heat or light.
Reactivity is a measure of how easily an element will combine with other elements to form compounds. Some elements are unreactive and need energy putting in others will react spontaneously and easily.
The periods of the periodic table are the rows that run from left to right in the periodic table. The elements are not arranged in these because of similar properties but because of their increasing atomic number from left to right.