Per this announcement on the WordPress.com weblog:
It’s easier than ever to add another dimension to your posts with embedded videos or tweets: announcing the Media Explorer, a new media curation tool that lets you add trending content from Twitter and YouTube without ever leaving your post editor!
The feature is nested in the Add Media button on the dashboard, and should be soon coming to wordpress.org blogs through the Jetpack plugin.
Here’s an example of a Tweet I found about the announcement.
Here’s a YouTube video using the same method:
This works similar to how Storify embeds Tweets and YT videos into a story, except WordPress.com users can’t embed Storifys into blog posts because of restrictions on use of iframes. It’s a welcome addition to the blogging tool chest on WP.com.
Here’s the WordPress video walkthrough on how to use this feature: