A signal is an electrical or electromagnetic current that is used for carrying data from one device or network to another.
It is the key component behind virtually all:
Communication
Computing
Networking
Electronic devices
A signal can be either analog or digital.
Techopedia explains Signal
Typically, a signal is created when a command or data is sent to a device. It has implementation in electrical and electronic components as well, but it mainly refers to analog and digital communication technologies and devices. Each signal carries data in some form. The data is fed into the signal using analog or digital modulation techniques, depending upon the source and destination device and/or medium.
Besides communication devices that broadcast a signal externally to the host system, signals are also used to communicate and send instructions by:
Processors
Memory
Storage
Numerous other components