It means that 306 sites have a link to yours. That's great, isn't it?
https://support.google.com/webmasters/answer/55281?hl=en tells how you can find them.
Alexa Traffic Rank show Sites Linking 306.
I'm including some other website link on my content that was included in 306 or not?
If answer is NO, so how to sites are linking with my website.