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The main idea is think of light as photons having E = hf. On a hot day, lots of photons are hitting me. The photons are all of different sizes. A large photon is the uv ray and the smallest photon is the Infra-red. These photons are hitting us like molecules except that the photons are energy particles that can be absorbed by our skin. So if these photons are incident on a metal surface the energy given to electrons enables it to be emitted having excess energy in the form of ke. The larger the photon the greater the ke. If photon energy is less than the energy required to release it from its bonds than no emission is possible. This minimum energy has a frequency that correspond to threshold frequency.
Analogy: You are like an electron. You would like to leave Singapore for a holiday . You would need a air ticket (say $1000). If you receive a packet of money $1500 , you would be able to pay for the ticket (to leave) and the remainder to spend $500. However , the packet of money received can be more than $1000 but not less than $1000. Hence $1000 is the minimum energy. Analogy stops here.
In photoelectric effect ,this minimum energy is called work function (corresponding f called threshold frequency). Lower energy (corresponding to lower f) no emission possible. The KE is known as KE max as some electron lose some energy due to collision as they are further from the surface and hence may have lower KE. Hence not all electrons have the same KE, thus there is KE max.