The oEmbed method of embedding content from other sites allows for "discovery", meaning that your WordPress install can figure out whether any given link can be embedded or not.
Now, this is inherently dangerous to allow any old site to embed in yours by simply posting a link, so this functionality is off by default. Embedding is limited to a list of specific sites which most people will want to use. Enabling "discovery" allows you to embed from anywhere that properly supports oEmbed.
So, use at your own risk. If you enable discovery, then make very sure you only link to trustworthy places.
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