If instead of just replicating, you want to connect containers running on different hosts to your Swarm infrastructure, you have to use networks. For example, you need to connect your web servers to your database containers so that they can communicate.
The answer to this, in Swarm Mode, is to use overlay networks. They are implemented with Docker's libnetwork and libkv. These networks are VxLAN networks built on top of another network (in the standard setup, the physical host network).
VxLAN is an extension of the VLAN protocol, aiming at increasing its scalability. Containers on different hosts, connected to Docker VxLAN networks, can communicate as if they are on the same host.
Docker Swarm Mode includes a routing mesh table that enables this multi-host networking, by default, called ingress.