I2P (Invisible Internet Protocol) is a universal anonymous network layer. All communications over I2P are anonymous and end-to-end encrypted, participants don't reveal their real IP addresses. I2P client is a software used for building and using anonymous I2P networks. Such networks are commonly used for anonymous peer-to-peer applications (filesharing, cryptocurrencies) and anonymous client-server applications (websites, instant messengers, chat-servers). I2P allows people from all around the world to communicate and share information without restrictions. For more information about I2P, you can read article on Wikipedia.