The existing answers aren't wrong, but I think the question hasn't been fully answered yet.
なくては usually means "without" in a broad sense. Its meaning is actually more neutral than the "unless" translation that user4092 mentioned, and is often a hypothetical condition. Example:
It's important to distinguish also between -ては and -て:
- 分からなくては心配です "Without understanding (i.e. unless I've understood), it's a worry"
- 分からなくて心配です "I'm worried about my not understanding"
In the second example, 分からない is an actuality, not just hypothetical like in the first example.
What does the verb after なくちゃ or なくては mean?
More generally, なくては means something like "[something] not being [the case] (at least), what follows will/did occur/is true/is my assertion." This is actually a very broad pattern: gerund + は. See chapter 22B of JSL (which I lifted the above examples from).
You can also follow with an adjectival, or any other predicate: 直さなくては危ないよ！
Even これじゃありません is an instance of this pattern (the gerund being で, where では is contracted to じゃ): this being the case/thing under consideration, it isn't that -> "as for this, it's not" -> "it's not this at least" -> "(it's) not this".
Also, as alternates for "must" there are patterns with と and with the provisional, which are beyond the scope of this question. See What is と in this sentence?
and Difference between ~なきゃ and ~なくちゃ