what is += (o, arg) => actually achieving?

Sorry to ask all, but I'm an old hand Vb.net guy who's transferring to c#. I have the following piece of code that seems to activate when the (in this case) postAsync method is fired. I just don;t understand what the code is doing (as follows):-

app.PostCompleted +=
    (o, args) =>
    {
        if (args.Error == null)
        {
            MessageBox.Show("Picture posted to wall successfully.");
        }
        else
        {
            MessageBox.Show(args.Error.Message);
        }
    };

if anyone could explain what the += (o,args) => is actually acheiving I'd be so greatful....

many thanks in advance. Tim