What is the meaning of the saying "It's not what you know but who you know?"
The meaning of this saying is that people get ahead in life based on their connections, not on their skills or their knowledge. It is saying that often people advance based on nepotism or on friendship, not on whether they actually know what they are doing in their job.
There are many examples of this, especially in countries where corruption is common. For example, it is said that Muammar Gadhafi's children are all rich and have important jobs simply because they are his children, not because of anything that they actually know. On a much smaller scale, you have probably seen this in your own neighborhood. High school students often get summer jobs, for example, based on who they or their parents know, not based on whether they would be a really good worker or whether they have a given set of skills.
So, the meaning of your saying is simply that people get ahead in life based on their connections (who they know) not based on their skills (what they know).