The second is defined as the duration of 9,192,631,770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. So what is meant by a cycle of radiation?
Taken a point in the space, hit by the radiation, the electric field in that point will oscillate periodically, as $\sin(\omega t)$ (where $t$ is the time). The oscillation will repeat exactly equally after a period $T=2\pi /\omega$.
The part of oscillation taking place during a time interval of length $T$ is "one cycle". It is, for example, the part of oscillation between two minima (or between two maxima).
A second is defined as the time needed to perform 9,192,631,770 cycles, i.e. 1 s=9,192,631,770 $T$.