If you're impressed, then you like what you see or you feel like saying, "wow!" People are impressed by brains, beauty, money, and lots of other qualities.
To be impressed, something has to leave an impression on you — it has to be special or unusually good or particularly emotional enough that you remember it or it stands out. Colleges are impressed by high school students who get straight A's. Some people are impressed by fancy cars and clothes. Lots of people are impressed by famous actors and sports stars. If someone isn't wowed by very much, we say they aren't easily impressed.