A hearing aid with a telecoil that is turned on can "hear" and amplify a magnetic signal that represents an audio signal. A loop is used to generate the magnetic signal.
A loop is driven by an amplifier, and the electrical signal, modulated to represent the audio signal, causes the loop to send out a magnetic signal.
The amplifier may be a powerful one that is part of an Assistive Listening System capable of driving a large room loop, or a small one that is part of a personal Assistive Listening Device capable of driving smaller loops like a neckloop or a silhouette.
That changing magnetic signal induces a sympathetic signal in the hearing aid's telecoil. That signal is then amplified so that the person wearing the hearing aid can hear the sound.
Loops come in all sizes: