Ganbare yo, is is the imperative form of the verb Ganbaru, (頑張る (がんばる)), to persist (do your best)--- I'm shortening the explanation of it to that, at least...
It's more of a command, such as You must do your best! The 'yo' is added as emphasis, so it's quite directly a command to whomever, to do their best...
Casual usage might be ganbatte... or ganbaru yo...