The story is that "Opportunity" is a man with a long beard and a bald head. When he knocks, and you answer the door, you'd better grab him by the beard before he turns around and you have nothing to grab.
it means that when ever you find a time or the oppertunity to be someone or to do something, don't miss it and wait for a better opportunity.
grab it and use it and don't wait for the time a better opportunity comes to you. may be the better chance will never come to you.