This rainbow pentagon thing is the logo of the Fediverse. It symbolises all the different networks like Pixelfed, Mastodon, PeerTube etc that connect together to form a larger overall network. People on Pixelfed can follow and interact with people on Mastodon because they're both part of the Fediverse.

More info about the Fediverse at https://fedi.tips/what-is-mastodon-what-is-the-fediverse

You can download high quality versions of this logo at https://commons.wikimedia.org/wiki/File:Fediverse_logo_proposal.svg

#Fediverse