"I'm drinking coffee" is in the present tense. Something that is being done now. i.e. "I'm drinking coffee now." "I drink coffee" is in the present continuous tense. - something that continues to be done regularly or habitually. e.g. "I drink coffee every Friday" "I drink coffee sometimes"

"I drink coffee" is a statement of fact, habit or a regular action. This is present simple. "I'm drinking coffee" means at this moment now. This is present continuous.