What is += operator doing in the provided C# code?
<p>What does this <code>+=</code> operator means in this code, is it a lambda? 
I read the MSDN document for the lambda but didn't find any thing about this <code>+=</code> operator, I will be thankful if someone explain it to me  </p>

<pre class="lang-c# prettyprint-override"><code>translateButton.Click += (object sender, EventArgs e) =&gt;
{
    translatedNumber = Core.PhonewordTranslator.ToNumber(phoneNumberText.Text);
    if (String.IsNullOrWhiteSpace(translatedNumber))
    {
        callButton.Text = "Call";
        callButton.Enabled = false;
    }
    else
    {
        callButton.Text = "Call " + translatedNumber;
        callButton.Enabled = true;
    }
};
</code></pre>