It's often used to refer to a small stream, creek, or river.
However, depending on context, it might also refer to:
* A specific place name: There are several places in New Zealand called "Wai-iti", such as a beach near Nelson and a bay in the Marlborough Sounds.
* A metaphorical concept: In some cases, it can refer to a small amount of water or a small body of water.
To understand the specific meaning of "wai-iti" in a particular situation, you need to consider the surrounding context.