A banner on gmail says this "This version of Safari is no longer supported. Please upgrade to a supported browser. Dismiss" what to do?

I'm not very tech savvy so please type replies in very easy language.

I use safari as my browser. Just updated my Apple software for my macbook Mac OS X version 10.6.8 (whatever that means) This banner recently appeared. When I clicked to see what browsers were supported - well hey there was safari! and an odd note that said if your browser is mentioned here then you probably have "browser extensions" attached that are not reocgnised so basically - go deal with them and you will be fine.

After a bit of internet hunting found out what a browser extension is - only have one and it has been working fine until just now - deleted it anyway bit that action has made no difference so now stuck.

Any ideas anyone?