The frequency of a wave is the inverse or reciprocal of its period. Period, by definition, is the time required for a wave to advance or vibrate through one complete cycle. It is measured in seconds. Frequency is how often the particles of a medium vibrate as a wave passes through, or how many times a wave cycles or repeats in a second. The unit for frequency, Hertz or Hz, means cycles per second.
This can be visualized by considering waves in water. Imagine a buoy bobbing up and down as a boat's wake causes mechanical waves to pass through water, the medium. The length of time between successive "bobs" of the buoy is the wave's period. The number of time the buoy bobs in one second is its frequency. Frequency therefore equals 1/period. When we say that an event happens "frequently", we are referring to how often it happens in a particular time period. When we say something occurs periodically, we're referring to the amount of time that has elapsed since it last occurred.