What's the meaning of it?

It is a slang way of saying that the desired result has been achieved.

eg...."If you need some pink paint, simply mix some red paint and white paint together, and Bob's your Uncle"

"Bob's your uncle" originated when Robert Cecil, the Prime Minister, appointed his nephew, Arthur Balfour, to a prestigious position.
The phrase was originally a sarcastic jibe at nepotism, but today simply means "no problem" or "everything is done".

See wikipedia....

http://en.wikipedia.org/wiki/Bob%27s_your_uncle