A cell is a group of servers and 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 cut off the network.
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. While it may be useful to also have the ability to route read traffic to individual cells, Vitess currently serves reads only from the local cell. Writes will go cross-cell when necessary, to wherever the master for that shard resides.