If you click the "bubble" icon on the numeric display widget added to your channel, you will get the HTML code required to embed the value on your HTML page. It should be of the form:
<iframe width="450" height="260" style="border: 1px solid #cccccc;" src="https://thingspeak.com/channels/<CHANNEL_ID>/widgets/<WIDGET_ID>"></iframe>
Where <CHANNEL_ID> and <WIDGET_ID> are to be replaced by those for your channel & widget. Note that you will need to make your channel public if you intend to embed the widget on your HTML page.