If you want to be able to place an IFrame in your page, and you use JCE Editor as your text editor, you must enable this feature.

Components > JCE Editor > Editor Profiles

Select the profile that you wish to allow iFrame embedding.

Go to the Plugin Parameters tab.

mediasupportIn the left hand column, click on Media Support.

To allow IFrames, select the Yes radio button.

Select Save & Close.

Go into the article where you want to place the IFrame.

Select Toggle Editor to get to the HTML for the document.

Put in the IFrame tag, per this example:

<iframe width="560" height="315" src="//www.youtube.com/embed/_WQDP4Ghp5Q?list=UUv2BcgqxHSrl2QQfIkjR5Eg" frameborder="0" allowfullscreen></iframe> 

It does not have to be a video, you can IFrame another web page, but videos are the most frequent reason to have an IFrame.You can change the width and height to fit into the area that you want.

Select Save & Close and test it on a couple different browsers.