c# syntax question

I am P/Invoking several functions. Looking at an atricle on how to do this I ran across the following syntax in the function declaration: [DllImport ("kernel32.dll")] internal static extern Boolean SetCommState(IntPtr hFile, [In] ref DCB lpDCB); and [DllImport("kernel32.dll")] internal static extern Boolean ReadFile(IntPrt hFile, [Out] byte [] lpBuffer, UInt32 nNumberOf BytesRead, IntPtr lpOverlapped); My question is about the [IN] and the [Out] syntax. What do they mean?? Thanks, Keith Stallings