The primary translation is sandwich. This is the one you should be using, unless the context give you reason to think otherwise. If there is no context, use the primary meaning. This isn't difficult.
Think of your translation with bread crumb: "you eat a bread crumb." Is this likely to be the information conveyed? Or is sandwich the more likely statement?
Duolingo wants to show you that this word does have other applications in other situations. Take it from someone who lives in Rome: if you talk about a panino, everyone is going to think you are talking about a sandwich.
Panino, as bread crumb, is the diminutive of bread. It can also signify a small bread loaf, or a roll. While actual usages of the word, almost no native speaker will hear that sentence above and think of anything else but sandwich.