なくてはならない

なくてはならない means "must", "have to": not doing it is not an option. Take the ない-form, change the final い to くて, and add はならない – 出さない becomes ださなくては出さなくてはならない. The polite form ends in なりません.

なくてはならない or なくてはいけない

The two mean the same. ならない is the more formal, and it suits rules, duties and things that hold for everyone; なくてはいけない is what you hear in everyday speech.

In conversation both shrink to なくちゃ, and なければ to なきゃ.

"Indispensable"

Built on ない itself, "there is not", なくてはならない describes something you cannot do without. It comes before a noun, with に for what needs it.