If something is indestructible, it is too strong to be destroyed — or that is the claim. Superheroes are called indestructible in stories; suitcase ads say “indestructible”; a parent laughs that a cheap plastic toy is indestructible because it has survived every drop. Though actually, in real life people use it loosely. They rarely mean “nothing in the universe could break this.” They mean “this thing will not break in normal use,” or even “this feeling will not die.” You can say an indestructible spirit or an indestructible habit. It is an adjective, a bit dramatic, fine in speech and in ads. For people, English more often says you cannot kill them (in fiction) rather than calling a person indestructible — though comics do exactly that.