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

Question

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