This a Type-Specifier function. It takes a floating-point number parameter (e.g. 57.35) and simply returns it. Its primary use is when you want to ensure that a data reference or prompt value is treated as being of type float in contexts where its role might otherwise be ambiguous.
Other type-specifier functions which can be used in a similar way are Number, Date, Text, Bool and Item.
=Float(["Enter floating point number"])