in many of the function of APIs I must sent recvwindow. so what is recvwindow?
An additional parameter, recvWindow, may be sent to specify the number of milliseconds after timestamp the request is valid for. If recvWindow is not sent, it defaults to 5000.
This is related to timing security. You can read about it in the link below.
https://binance-docs.github.io/apidocs/spot/en/#signed-trade-user_data-and-margin-endpoint-security