As an extension of the ‘if’ meaning of
と, attaching the auxiliary verb ない to the verb before it will express ‘if (A) is not done, (B)’. (B) is most frequently いけない ‘cannot go’, which makes
ないといけない a double negative phrase, which translates to ‘must do (A)’, or ‘have to do (A)’.
いけない may be replaced with だめ, to make the phrase more casual, but ならない is generally not used with
ないと.
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ライト君（くん）が悪（わる）いんだから、リュウ君（くん）に謝（あやま）らないといけないでしょう。
Raito-kun, you are in the wrong, so you have to apologize to Ryu-kun, right?
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今日（きょう）は勉強（べんきょう）しないとだめだから、明日（あした）は？
I have to study today, so how about tomorrow?
ないといけない is regularly abbreviated to
ないと, but has exactly the same meaning in these cases.
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明日（あした）はテストがあるの？！勉強（べんきょう）しないと。
There is a test tomorrow!? I have to study.
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もう7時（じ）じゃん？準備（じゅんび）しないと。
Oh, it's already 7? I need to get ready.
As
と has the ‘and’ nuance, it gives
ないと a level of strength that is not seen in similar phrases like
なければいけない, or
なくてはいけない. Because of this,
ないと is often used to highlight things which absolutely ‘must’ be done (obeying the law, adhering to customs, using common sense, etc.).