I learned it as "rubor, tumor, calor, dolor, et functio laesa" meaning redness, swelling, heat, pain and loss of function. These are the classic signs and the reason it gets its name, as it's like a fire in the body. But as other commenters have said, it now seems to be the presence of certain signalling molecules or cell types that are associated with inflammation, and when people speak of it they may not be referring to any of the classical signs of inflammation. Which is a shame because now its lost the helpful etymology.

I'd like a physics level explanation.. when those molecules are there? whats going on at the atomic level, is there an excess of negatively or positively charged molecules ? When there is inflammation, there is usually heat, and heat means energy.

Lots of things can be going on. One of the most dramatic things is the respiratory burst of neutrophils. https://en.wikipedia.org/wiki/Respiratory_burst. But neutrophils are not the only inflammatory cell and not all inflammatory processes involve them. There is way too much to describe in a comment on the internet.