A current loop is an electrical setup that uses two conductors to send signals to a device. It can be used for analog signaling or supplying power.
Techopedia explains Current Loop
In many cases, the purpose of a current loop is to analyze or measure things like temperature, pH value or other variables in an environment. It can also be used for things related to grounding and other system elements.
The devices connected to a current loop may be active and have their own power source, or they may be passive and rely on power from the loop.
There are different types of current loops, including long telecom line loops, discrete control functioning loops and other kinds of process controllers.
This type of electrical engineering has been used extensively in telephony and in two-way radio systems.
A current loop can often be easily monitored using a multimeter.