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)

Unlike in C, in Go assignment between items of different type requires an explicit conversion. Try removing the float64 or uint conversions in the example and see what happens.