Spectrum 2 is an open source instant messaging transport. It allows users to chat together even when they are using different IM networks. It acts as a transport layer between the users as showed in the animation above.
If some network is supported as **Frontend**, it means that Spectrum 2 allows its users to use this network to communicate with other users using the **backend** network.
If you for example use **Slack** as a frontend and **IRC** as a backend, the Spectrum 2 users can add Spectrum 2 to their Slack team, join the IRC network and communicate with their friends on IRC network.