To advise someone is to give them advice, or counsel. If you live in Iceland and a friend visiting you from Hawaii asks what he should pack, advise him to bring warm clothes and snow boots.
Advise comes from a Latin word for vision. We want people to advise us if we think they can see the whole picture better than we can. For this reason, we often seek the advice of trained people. Financial advisors advise their clients on which funds to invest in. Legal advisors advise their clients on the law. To advise can also be to give notice. You should advise your boss of your decision to quit at least two weeks in advance.