Former refers to something that came at an earlier time, or before something else. For example, someone who believes in reincarnation might wonder about their former life — or even lives — before their present existence.
When something or someone has changed, we can refer to them as former. For example, Mr. Smith's ex-wife could be known as the former Mrs. Smith. We could consider almost anything as former, as long as it comes first, before something else. A good way to remember the meaning of former is to consider that it comes from the Old English forma, which means "first," and that both former and first begin with "f."