eattherich.club hosts an instance of the open source Mastodon project backed by the ActivityPub protocol. That may sound intimidating at first but don't let that fool you, as Franklin López put it in his How to Use Mastodon video, "It's pretty fucking simple." Just like every other platform you can create text, photo and video posts, unlike other platforms these posts can be viewed by any other ActivityPub instance. This means much like email you could host your data on eattherich.club and still interact with people who host their data on kolektiva.media by simply posting "Loved your video! @firstname.lastname@example.org"
This network of interacting instances is known collectively as The Fediverse, thousands of independently run servers with small (sometimes not too small) communities that co-mingle on the federated timeline.
Finally, it is important to note that there are many ways to join The Fediverse. eattherich.club is just one option. You may also be interested in Pixelfed or PeerTube which also use ActivityPub but are focused more on photos and videos respectively. You may even like to browse for different Mastodon instances to join (we won't be too sad, that's the beauty of federation!)