golang的zlib / reader.go文件中的“ r。（flate.Reader）”是什么意思？

I found lots of code snippets like r.(flate.Reader) in golang's zlib/reader.go file. What does it mean?
https://golang.org/src/compress/zlib/reader.go

func (z *reader) Reset(r io.Reader, dict []byte) error {
    if fr, ok := r.(flate.Reader); ok {
        z.r = fr
    } else {
        z.r = bufio.NewReader(r)
    }
    // more code omitted ...
}

P.S. source code of io and flate.
io: https://golang.org/src/io/io.go
flate: https://golang.org/src/compress/flate/inflate.go

The Go Programming Language Specification

Type assertions

For an expression x of interface type and a type T, the primary expression

x.(T)

asserts that x is not nil and that the value stored in x is of type T. The notation x.(T) is called a type assertion.

A type assertion used in an assignment or initialization of the special form

v, ok = x.(T)
v, ok := x.(T)
var v, ok = x.(T)

yields an additional untyped boolean value. The value of ok is true if the assertion holds. Otherwise it is false and the value of v is the zero value for type T. No run-time panic occurs in this case. C

r.(flate.Reader) is a type assertion. For example,

func (z *reader) Reset(r io.Reader, dict []byte) error {
    if fr, ok := r.(flate.Reader); ok {
        z.r = fr
    } else {
        z.r = bufio.NewReader(r)
    }
    // more code omitted ...
}

r is type io.Reader, an interface. fr, ok := r.(flate.Reader) checks r to see if it contains an io.Reader of type flate.Reader.