It’s a heavily modified firefox browser designed to work with something called “the onion network”. It’s called this because there are several nodes on the network designed to obfuscate your Internet traffic by wrapping a layer on your Internet traffic, creating an “onion”. All of these layers mean that each node only knows what the previous and next nodes are. The most vulnerable nodes are the starting and exit nodes, because they can identify you and potentially trace back your IP. You also can’t choose your starting or exit nodes. It’s well known that the US federal government controls some of these exit nodes.