what is meant by [DllImport("user32.dll") ?

It means that the method declared below it is not in .NET - it is in a external (native) DLL file.
In this case, it is in the User32.dll file, which is a standard Windows component.

For example:

[DllImport("User32")]
private static extern int SetForegroundWindow(IntPtr hwnd);

The method SetForegroundWindow is a standard Windows function, and teh above declaration allows us to us it as if it was a .NET method:

public static void SingleInstance(this Process thisProcess)
    {
    foreach (Process proc in Process.GetProcessesByName(thisProcess.ProcessName))
        {
        if (proc.Id != thisProcess.Id)
            {
            ShowWindow(proc.MainWindowHandle, SW_RESTORE);
            SetForegroundWindow(proc.MainWindowHandle);
            thisProcess.Kill();
            }
        }
    }