This is a rather personal question, and I can't answer for you, but I can give you my opinion.
When Macbeth says that line, he is referring to the meaninglessness of life. He is saying that life isn't really worth living because it is just a fleeting moment in time, and will eventually end, no matter what you do about it. He tried hard to fight for his ambition and become a king, and he did, but he died nevertheless, thus reaffirming his belief that life is 'signifying nothing' in the end.
Personally, I disagree with his opinions. Yes, life is going to end and we aren't going to live forever, however, we have to make do with what we were given. We aren't supposed just to lie down and wait to die - we should live our life to the fullest and make the most of it while we still can. As the famous aphorism goes, Carpe diem (Seize the day)!
