Html head meta tags explanation required? : <p>I have seen the following meta tags in many places can anyone explain that are these necessary and how? What do they exactly mean?</p>

<pre><code>&lt;meta name="hdl" content=""&gt;
&lt;meta name="lp" content=""&gt;
&lt;meta name="byl" content=""&gt;
&lt;meta name="utime" content=""&gt;
&lt;meta name="ptime" content=""&gt;
&lt;meta name="pdate" content=""&gt;
&lt;meta name="dat" content=""&gt;
&lt;meta name="CG" content=""&gt;
&lt;meta name="SCG" content=""&gt;
&lt;meta name="PT" content=""&gt;
&lt;meta name="PST" content=""&gt;
&lt;meta name="CN" content=""&gt;
&lt;meta name="CT" content=""&gt;
&lt;meta name="genre" itemprop="genre" content=""&gt;
&lt;meta name="url" itemprop="url" content=""&gt;
</code></pre>
