定义在别处： Friend WithEvents SerialPort1 As System.IO.Ports.SerialPort
然后就是
Private Sub SerialPort1_DataReceived(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) Handles SerialPort1.DataReceived
Control.CheckForIllegalCrossThreadCalls = False
Dim SerData, S1Temp As String
SerData = SerialPort1.ReadTo(" ")
SerData = SerData.Trim()
其中SerData = SerialPort1.ReadTo(" ") 这句是做什么的，是读串口的数据吗？

读取串口中的数据直到遇到空格（包括空格），然后赋给SerData

要学会查msdn
https://msdn.microsoft.com/zh-cn/library/system.io.ports.serialport.readto.aspx