golang struct 定义中的 chan 词是什么意思

我正在阅读以太坊代码，以下是什么意思

<-chan struct{}和chan<- error？

type protoRW struct {
   Protocol
   in     chan Msg        // receives read messages
   closed <-chan struct{} // receives when peer is shutting down
   wstart <-chan struct{} // receives when write may start
   werr   chan<- error    // for write results
   offset uint64
   w      MsgWriter
}