The basic principles are simple: stuff goes in from the Sun and the inside of the Earth; stuff goes out through the atmosphere. Greenhouse gases trap stuff in the atmosphere, which results in the accumulation of stuff in the oceans and atmosphere. No need to delve into entropy and free energy here: the concept of heat is enough*.
The details are insanely complex and I would not claim to understand the beginning of it, though.
* the concept of heat is of course horrendous from a scientific point of view and we do use things like enthalpy and entropy in modern thermodynamics. But there’s a reason why heat is used in vulgarisation and introductions. It is intuitive and useful if you don’t dig too far, so perfect to explain things to laymen and 5 years olds.
