The conditional form of 'nai' indicating 'if (it) doesn't exist' or 'if (one) doesn't have ...' (i.e., 'if there is no ...').
お金がなければ旅行に行けない。
If I don't have money, I can't go on a trip.
情報がなければ正しい判断はできない。
Without information, you can't make a correct judgment.
時間がなければ後日に延期しましょう。
If there isn't time, let's postpone it to another day.
Used like 'unless...' or 'if not...' to express a negative conditional (i.e., 'if not ...').
連絡がなければ、出発します。
If I don't hear from you, I'll depart.
説明がなければ誰も分からないだろう。
Unless there is an explanation, no one will understand.
雨が降らなければ明日ピクニックに行く。
Unless it rains, we'll go on the picnic tomorrow.
An abbreviated expression for obligation ('must' or 'have to') as in '〜なければ（ならない）'. In speech, '〜なければ' alone often implies 'must ...'.
早く行かなければ。
I have to go soon.
宿題を終わらせなければならない。
I have to finish my homework.
この書類は今日中に出さなければ（ならない）。
This document must be submitted by today.