What did Jimmy mean when he said Bud better be careful with his instrument because that was bread and butter in there Not Buddy?

In the context of the given sentence, Jimmy is likely referring to the fact that Bud's instrument is his livelihood and source of income. The expression "bread and butter" is commonly used to describe something that provides sustenance or financial support. So Jimmy is essentially telling Bud to take good care of his instrument because it's what puts food on the table and allows him to make a living.