The full question is:
On a keyboard, you strike middle C, whose frequency is 256 Hz. What is the period of one vibration of this tone?
The period of a vibration is the time it takes for the particle to make one full oscillation. Frequency is by definition number of full oscillations per unit of time.
When the frequency is expressed in Hz that unit of time is one second.
So there is the following relation between frequency and period:
[tex]T=\frac{1}{f}[/tex]
When we plug in the numbers we get:
[tex]T=\frac{1}{256}=0.0039$s[/tex]
