When you're around enormously successful people you realise their success isn't an accident - it's about work.

This quote is suggesting that when you spend time with people who have been very successful, you understand that they didn’t become successful by luck or chance, but rather by working very hard at their goals. They put in a lot of effort and it pays off. The success they have achieved is no accident and can be achieved by anyone who is willing to devote themselves to their goals.