It’s been a while since I used Office 2010 and so if someone has it installed, please step in.
If you create rules like ‘If mail received from’ [email account] ‘then move it to’ [Folder] or something like that, they normally executes when you receive mails.
However, when you create a rule, chances are that you already have lots of messages which matches that rule in your inbox. In this scenario, ‘Run Rules Now’ could be used to make sure (using the example) all existing messages from that email account would be moved to the folder.
Hope it answers your question