What do you understand by resonance? How would you use resonance to determine the velocity of sound in air?

When the frequency of external periodic force acting on the body is equal to the natural frequency of that body, the body vibrates with maximum amplitude. This phenomenon is called resonance.
Experiment :

When natural frequency of air column coincides with the frequency of vibrating tuning fork, the air column would be in resonance with the tuning fork. Such an air column is called resonating air column.
When the tuning fork is vibrated on the air column then the first sound is heard at a length of air column ${\mathrm{l}}_1=\mathrm{\lambda }/4$ and the second sound is heard at the length of the air column is ${\mathrm{l}}_2=3\mathrm{\lambda }/4$. We observe that ${\mathrm{l}}_2=3{\mathrm{l}}_1$.
$\begin{array}{l}{\mathrm{l}}_2-{\mathrm{l}}_1=3\mathrm{\lambda }/4-\mathrm{\lambda }/4=\mathrm{\lambda }/2\\ \mathrm{\lambda }=2\left({\mathrm{l}}_2-{\mathrm{l}}_1\right)\end{array}$
The velocity of sound in air is determined from equation $\mathrm{V}=\mathrm{n\lambda }$
$\therefore \mathrm{V}=2\mathrm{n}\left({\mathrm{l}}_2-{\mathrm{l}}_1\right){[}^{\text{'}}\mathrm{n}\text{ ' }$is called frequency of tuning fork].