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