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.
你 要不 看书 要不 听音乐 。
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.
要不 我们 去 逛街 吧，比 在家 睡觉 有 意思。
要不 means otherwise
要不 can also be used to mean otherwise:
我 要 打的, 要不 我会迟到。
要不然 can be used interchangeably with 要不 when it is used as 否则.