Is to talk to, or to mention another user, here called somethingsomething.
Is to tag your twit with the tag #somethingsomething, so people can click on the tag and find out who else is talking about #somethingsomething.
For instance when AbeMudokon posts:
:
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All'ya word in the farm is there will be some big news on Monday. I'm gonna follow @alfmudokon to find out. #Oddworld
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He's referring to the user alfmudokon and tagging his tweet with the #Oddworld tag.