Expresses that something is indispensable or cannot be done without — colloquial shortened form of なくてはいけない.
パスポートがなきゃいけないので、早めに準備してください。
You need a passport, so please prepare it in advance.
医者の診断書がなきゃいけない場合もある。
There are cases where a doctor's certificate is required.
そんな高い機材がなきゃいけないわけじゃないよ。
It's not that you absolutely need such expensive equipment.
Indicates obligation or necessity: 'have to' / 'must' — used after the negative stem of a verb (e.g., 行かなきゃいけない 'have to go').
明日早いから、もう寝なきゃいけない。
I have to go to bed now because I have an early start tomorrow.
レポートを出さなきゃいけないので、徹夜します。
I have to turn in a report, so I'll stay up all night.
忘れ物があるから、取りに戻らなきゃいけない。
I left something behind, so I have to go back and get it.