The more o's used in the word, the better the thing is for example:
"she's hoora fat" might mean she is quite fat but "she's hooooooooooooooooora fat" means that she is clinically obese!
As this is a highly localised word, not present in standard English or widely-recognisable Scots, there is no universally accepted spelling, with variants such as "hura" or "huura" common. Generally, the more vowels included, the more intense the meaning (in both spoken and written communication).
"Hoora" is often used as equivalent to "very", but is flexible and depending on context can replace most any adjective the speaker wants it to. In this way hoora is related to words such as "hella" and "wicked" in the USA. It can also be used as a noun to signify a large number.