The word what is that is one of the most useful in the English language. It is used when you want to ask your audience to identify an object. It can be used with almost any object and it also carries a sense of informality and humour as it is using the audience’s voice instead of yours.
You can use it in conjunction with other words to convey meanings such as that is, that isn’t, or that will. It can also be used in the context of finality or closure, for example when a decision has been made and there is no more discussion to be had about it.