要不 (yào bù) can have a number of meanings depending on how it is used. It means giving alternatives, Indicating cause and effect

要不 as to mark possible choices

要不 can be used to offer "either/or" choices to others, similar to 或者.

Nǐ yào bú kàn shū yào bú tīnɡ yīn yuè.
你 要不 看书 要不 听音乐 。
You can either study or listen to music.

Another word that can be used in a similar pattern is 要么, with the difference that 要么 can only be used to list the either/or choices available to oneself as well as the choices available to others.

要不 used for suggestions

要不 can be used as an indirect way of giving a suggestion, as the speaker is using 要不 to seek consent of the listener.

Yào bú wǒ men qù ɡuànɡ jiē bɑ , bǐ zài jiā shuì jiào yǒu yì si .
要不 我们 去 逛街 吧，比 在家 睡觉 有 意思。
Let's go shopping.It's more fun than sleeping at home.

要不 means otherwise

要不 can also be used to mean otherwise:

Wǒ yào dǎ de, yào bú wǒ huì chí dào.
我 要 打的, 要不 我会迟到。
I'm going to take a taxi, otherwise I'll be late.

要不然 can be used interchangeably with 要不 when it is used as 否则.