A cell is a group of servers and associated network infrastructure collocated in an area, and isolated from failures in other cells. It is typically either a full data center or a subset of a data center, sometimes called a zone or availability zone. Vitess gracefully handles cell-level failures, such as when a cell is isolated from other cells by a network failure. A useful way to think of a cell is as a failure domain.
Each cell in a Vitess implementation has a local Topology Service, which is hosted in that cell. The Topology Service contains most of the information about the Vitess tablets in its cell. This enables a cell to be taken down and rebuilt as a unit.
Vitess limits cross-cell traffic for both data and metadata. Vitess currently serves reads only from the local cell. Writes will go cross-cell as necessary, to wherever the master for that shard resides.