According to Einstein's photoelectric equation

${\text{K}}_{\text{max}}=\text{h}\upsilon -\text{h}{\upsilon }_0$

Since ${\mathrm{K}}_{\max }$ is  positive, the photoelectric emission occurs only if

$\text{h}\upsilon >\text{h}{\upsilon }_0\text{or}\upsilon >{\upsilon }_0$
 The photoelectric emission occurs only when the incident light has more than a certain minimum frequency. The minimum frequency is called threshold frequency.