Extremely means to a very great degree or far beyond what's usual. When something is extremely hot, it's scorching, much hotter than warm or even regular hot. When a problem is extremely difficult, it's much harder than a typical challenge.

The word intensifies whatever it describes. A tall building becomes an extremely tall skyscraper. A fast runner becomes an extremely fast sprinter. Good ice cream becomes extremely good ice cream (the kind you think about days later).

You can use extremely to emphasize almost any descriptive word: extremely happy, extremely tired, extremely careful, extremely messy. It sits at the far end of the scale. If “very” turns the volume up to 7, “extremely” cranks it to 10.

If everything becomes extremely important or extremely exciting, the word can lose its punch. Using it when something genuinely stands far above the ordinary helps people know you're serious when you say something is extremely interesting or extremely urgent.