"He eats an egg."
Translation:Él se come un huevo.
se is a reflexive term/word, meaning the action reflects the person that is doing it. in this case, [the boy] "he" is the one eating the egg.
A better example for "se" would be: se lastimo. meaning, HE hurt HIMSELF.
I think uno refers to the number 'one' while un is the adjective one before the object