A more thorough explanation:

Surveillance is when someone watches another person to gather information. Police often use surveillance to investigate suspects and collect evidence. There are two types of surveillance: overt and covert. Overt surveillance is when the person being watched knows they are being watched. Covert surveillance is when the person being watched does not know they are being watched.

Surveillance can be done in different ways. Electronic surveillance is when technology is used to watch someone. Examples of electronic surveillance include wiretapping, bugging, and tracking someone's location using their phone. Fixed surveillance is when someone is watched in person. This is also called a "stake-out."

However, there are limits to surveillance. The Fourth Amendment of the U.S. Constitution protects people from unreasonable searches and seizures. This means that surveillance cannot be done without a good reason. For example, in the case of Kyllo v. U.S., the Supreme Court said that using technology to look inside someone's home without a warrant is not allowed. Electronic surveillance also requires a warrant, which is a legal document that allows the police to do the surveillance. The Electronic Communications Privacy Act (ECPA) is a law that regulates electronic surveillance and gives people the right to sue if they are illegally watched.

For example, if the police want to listen to someone's phone calls, they need a warrant. They have to show that they have a good reason to listen to the calls, and they have to describe exactly what they will be listening to. This is to make sure that the police do not listen to calls that are not related to their investigation.