When something is terminable, it comes to an end rather than going on forever. In law, terminable describes an agreement that can be ended. If you want to pursue a solo singing career, you'd better hope your contract with your band's manager is terminable.
If an apartment lease is terminable, that means you're allowed to break it or get out of it under certain circumstances. And if the song your little brother sings is terminable, it eventually comes to an end—this sense of the word is less common than its antonym, interminable, "appearing to be infinite." Both words come from the Latin root terminus, "end or limit."