Questions & Answers about El carbón ya está listo.
Because carbón is being treated as a specific thing: the coal / the charcoal.
In Spanish, articles are often used more frequently than in English. So where English might sometimes say coal is ready, Spanish will very naturally say el carbón está listo when referring to the coal being used in a particular situation, such as for a barbecue or fire.
Also, carbón is a masculine singular noun, so the article is el, not la.
It can mean either, depending on context.
In everyday situations, especially if people are talking about a grill or barbecue, el carbón often means charcoal. In other contexts, it can mean coal in the general fuel/mineral sense.
So the exact English translation depends on the situation, even though the Spanish word stays the same.
The accent mark shows where the stress goes: car-BÓN.
Without the accent, standard Spanish stress rules would make you expect the stress somewhere else. The written accent tells you clearly that the final syllable is stressed.
It also helps distinguish the correct written form of the word.