make(chan string) 这是什么意思啊？

``` package main import ( "fmt" "io" "io/ioutil" "net/http" "os" "time" ) func main(){ start := time.Now() ch := make(chan string) for _,url : range os.Args[1:]{ go fetch(url,ch) } for range os.Args[1:]{ fmt.Println(<-ch) } fmt.Printf("%.2fs elapsed\n",time.Since(start.Seconds()) } func fetch(url string,ch chan<- string){ start := time.Now() resp,err := http.Get(url) if err != nil{ ch <- fmt.Sprint(err) return } } ``` + make(chan string) 是什么意思 + func fetch(url string, *** ch chan<-string ***){ 这又是什么意思