Unbreakable means it cannot be broken. The plain physical use is about objects that survive being dropped or hit: unbreakable glass, unbreakable plates for small children, an unbreakable phone case. Be honest with students about how this is really used — much of the time it is advertising language, and 'unbreakable' in a shop means 'very hard to break', not literally impossible. The second use is just as common and more interesting: things that are not objects at all but that we imagine as things that could snap. An unbreakable promise, an unbreakable bond between two people, an unbreakable habit, an unbreakable record in sport (nobody will ever beat it), an unbreakable code (nobody can read it). In each case English is treating a promise or a friendship like a stick or a rope. There is also 'unbreakable spirit' — a person who cannot be crushed. The word is ordinary in level, at home in both writing and speech.