The Fediverse is a network of different servers that share their resources so that you can can find other people and exchange ideas with them. In principle, it does not matter which of the platforms you use. A platform is the software that provides you with a user account and provides access through a user interface.
Platforms communicate with each other using protocols. Protocols are defined information about how text, images, and many other attributes must be handled to display them on any platform.
The common protocol ensures that the message sent by Peter (e.g. on Mastodon) through Petra (on Friendica) can be received and displayed. Both can comment and exchange pictures or videos using the same protocol.
The most widely used protocol is called ActivityPub. It was defined by the World Wide Web Consortium (W3C) on January 23, 2018, as the official standard for decentralized social networking. However, the Fediverse speaks other protocols such as ZOT, DFRN, OStatus or diaspora*