$$\require{cancel}$$
$f = \frac{1}{2l} \sqrt{\frac{T}{u}}\nonumber$
where $$f$$ is the frequency in hertz, $$T$$ is the tension in kilogram meters per second squared (newtons), u is the mass per unit length of the string in kilograms per meter, and l is the vibrating length of the string in meters. In this experiment, the tension, $$T$$, is simply the product of a suspended mass (kilograms) times the acceleration due to gravity (9.8 meters per second squared). That is, tension is a force and force is equal to mass times acceleration. Note that longer strings and more massive strings produce a lower frequency of oscillation and that increased tension increases frequency.