What does it mean if staphylococcus aureus got on my skin and backpack?

Too late. Staphylococcus aureus is one of the most common bacteria there is on the planet earth. There is no question that it can be found on almost every surface including the two you mentioned. The question is, does the mere presence mean anything? The answer is no. Unless you have a bad immune system, or one of the more serious subtypes, the mere presence means you are alive here on earth with me.