Extremely is a word that emphasizes the intensity of whatever it is you're talking about. If you say you're extremely angry, that word alone tells me I better not mess with you.
Extremely comes from the word extreme, which means great, severe, or exceptional. If you climb to extreme heights on a mountain, you've climbed extremely high. Use extremely to let people know that what you're talking about exceeds or goes beyond the norm. A joke that's not just funny, but extremely funny, is one that might have you gasping to catch your breath from laughing so hard.