要不 (yào bù) can have a number of meanings depending on how it is.

要不 as to mark possible choices

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

Nǐ yàobú kànshū yàobú tīnɡyīnyuè.
你 要不 看书 要不 听音乐 。
You can either study or listen to music.

Yàobú nǐ zuò, yàobú wǒ zuò, dànshì yídìnɡ yào zuò wán.
要不 你 做，要不 我 做，但是 一定 要 做 完。
Either you do it or I do it, but it's definitely going to be done.

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.

Yíɡèrénqùkàndiànyǐnɡméiyìsi, yàobú wǒmen yìqǐ qù bɑ.
一个人去看电影没意思，要不 我们 一起 去 吧。
It's no fun to go to the movie by myself.How about we go together?

Yàobú wǒmen qù ɡuànɡjiē bɑ, bǐ zàijiā shuìjiào yǒu yìsi.
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:

Nǐ kuàidiǎn dǎdiànhuà ɡěi tā, yàobú tāhuì hěn dānxīn.
你 快点 打电话 给 他，要不 他会 很 担心。
I need to call him soon　or he'll start to worry.

Wǒ yào dǎde, yàobú wǒhuìchídào.
我 要 打的, 要不 我会迟到。
I'm going to take a taxi, otherwise I'll be late.

Wǒmen jīn wǎn xūyào shōushi hǎo xínɡli, yàobú mínɡtiān lái bú jí.
我们 今 晚 需要 收拾 好 行李，要不 明天 来 不 及。
We need to prepare the luggage tonight, otherwise there won't be time tomorrow.

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