It's a shortened and informal way of saying "for sure" or "for certain".
Here are some examples of how it's used:
* "Are you coming to the party?" "Fosho!"
* "I'm gonna get that new game." "Fosho, it looks awesome!"
* "That movie was really good." "Fosho, it was the best!"
It's a casual and friendly expression that adds a touch of local color to the conversation.