A sleeper agent is a spy or operative who is placed in a target country or organization, not to undertake an immediate mission, but instead to act as a potential asset on short notice if activated in the future.
The agent acquires jobs and identities, ideally ones that will prove useful in the future, and attempts to blend into everyday life as a normal citizen.
Counterespionage agencies in the target country cannot, in practice, closely watch all those who may possibly have been recruited some time before.
In a sense, the best sleeper agents are those who do not need to be paid by the sponsor, as they are able to earn enough money to finance themselves, averting any possibly traceable payments from abroad.
This common use is directly related to and results from repeated instances of real-life "sleeper agents" participating in spying, espionage, sedition, treason, and assassinations.