To be nice is to be pleasant and good-natured. Polite people and sunny days are nice.
Nice people and situations are enjoyable and don't cause problems. If you say something rude (or honest) to your sibling, your parents might say "Be nice!" This word is a little vague and overused. Like interesting, it's hard to know what people really mean when they say nice. One meaning is easier to figure out: if you score a goal in hockey, that was a nice shot. That means you were skillful and did well — you shot the puck nicely.