Photoelectric emission occurs only when the incident light has more than a certain minimum

According to Einstein's photoelectric equation $K_{max}=h\nu -h{\nu }_0$

Since $K_{max}$ is positive, the photoelectric emission occurs only if $h\nu >h{\nu }_0$ or $\nu >{\nu }_0$

The photoelectric emission occurs only when the incident light has more than a certain minimum frequency. This minimum frequency is called threshold frequency.