Sami is a decentralized communication application.
Said otherwise, it is an instant messaging app, like Whatsapp, except communications do not rely on centralized servers (like Facebook's in the case of Whatsapp), and therefore have some advantages over its centralized counterpart !
See section What is decentralization and why use it?
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Decentralization is a way of interconnecting users, as much as centralization is, though the two are often opposed.
A common centralization model on the Internet is the client-server model.
In this scenario, all clients - your computer and those of your friends - are connected to a single server. If Alice wants to send a message to Bob, Alice sends her message to the server, which relays it to Bob, and vice-versa. From a technical point of view, this is pretty handy and allows for great control, which is part of the reason it is so common.
Peer-to-peer on the other hand interconnects all users. If Alice wants to send a message to Bob, she sends it directly to his computer !
While it looks straightforward, in practice it is harder to implement than a client-server model. It's counterintuitive, yes, but like most things, there are historical reasons explaining this. Mainly - though indirectly - Capitalism, I believe ;)