Can you explain evolution to me please? Does it mean that an species will grow change to adapt to surroundings?

Like say a species needs to reach food will it's neck grow longer or something? I have read about Darwin's finches but don't get it. The finches are different yeah but they are still finches. They didn't turn into eagles or blue jays. Please forgive me if I sound ignorant but i have looked it up and still don't get it.