Shown is a sinusoidal wave as a function of time $$t$$. The frequency $$f$$ indicates how often a period $$T$$ fits into one second. In the presented sine wave, 2 and 1/2 periods occur within one second, therefore the frequency is $$f = 2.5 \, \frac{1}{\mathrm s} = 2.5 \, \mathrm{Hz}$$.