From Academic Kids
A covert cell structure is a method for organizing undercover or unconventional fighters against a large and well-established organization. If the covert cell is part of a resistance movement it can also be called a resistance cell, and if it's part of a terrorist organization it can be called a terrorist cell; in some cases, a given covert cell could be called both depending on who is asked.
The organizational structure of covert cells is intended to limit the harm that can be done if members are captured and interrogated. Most members will only know the identities of other people in their own cell; only the leader of a cell will know the identities of leaders of other cells and communicate with them. By keeping cell size small, captives or double agents will have a very limited knowledge of the organization as a whole.