It is called type assertion. Here is how it looks like:

str := value.(string)

It gives you access to the interface’s concrete value. It asserts if the values stored in x is of type T and that x is not nil.

Type assertion can also return two values: the concrete underlying value and a boolean value that check if the assertion is true.

t, ok := x.(T)

Here is some code examples for a much easier references. It is a similar example as the one you can find in A Tour of Go

    var i interface{} = "hello"

    s := i.(string)
    fmt.Println(s)
    // Will prints out: hello

    s, ok := i.(string)
    fmt.Println(s, ok)
    // Will prints out: hello, true

    f, ok := i.(float64)
    fmt.Println(f, ok)
    // Will prints out: 0, false

    f = i.(float64)
    fmt.Println(f)
    // Will cause "panic"
    // panic: interface conversion: interface {} is string, not float64

Additional references: