The expression T(v) converts the value v to the type T. Some numeric conversions:
var i int = 42
var f float64 = float64(i)
var u uint = uint(f)
Or, put more simply:
i := 42
f := float64(i)
u := uint(f)
Go assignment between items of different type requires an explicit conversion which means that you manually need to convert types if you are passing a variable to a function expecting another type.