If you were standing on a dock and water waves were coming at you, the period would be the time interval between waves. In other words, the period of the wave would be the time interval between the arrival of one wave crest and the next.
The period of a wave is closely related to its frequency. Frequency is the number of repeated motions in a period of time. These motions can be up and down, like the crests of water waves, or back and forth, like the waving of a hand. Or some other repeated motion, maybe the swinging of a pendulum. Repeated motions are called “oscillations” or “cycles.” Frequency is the number of cycles per unit of time. Often the unit of time is seconds.
Frequency and period are inversely related. The higher the frequency, the shorter the period; that is, the shorter the interval between motions. The lower the frequency, the longer the period.
Cycles can be graphed on paper, as in the accompanying image. In the case of the up and down motion of the water wave, the vertical axis would be elevation and the horizontal axis is time. When the wave is highest in elevation, it’s a crest. When it’s lowest, it’s a trough. Then, over time, it rises again in elevation to a crest, and so on. The period of this wave is the time elapsed from one crest (or trough) to the next.
Note: Sometimes physicists use the term “period” to mean the spatial distance between waves. Of course, outside the technical literature, the spatial distance between waves would be called “wavelength.” Frank Wilczek, a Nobel Laureate physicist, writes, “Usually the term [period] refers to repetition in time, though in the scientific literature it is also often used for repetition in space.” A Beautiful Question; Penguin Press, 2015, New York; p. 375 in the section “Terms of Art.”