Brands can invite you to a campaign if they think you're a good fit for the gig. Any gigs that you're invited to will show up under the "Invites" section of your app. We'd recommend applying to the gig or letting them know you're not interested. After all, it's always polite to RSVP.
Note: an invite is not a guarantee that you'll be selected for the gig, but, hey, the brand did invite you, so it may increase your chances of selection.