This is a tiny coil of wire within a hearing aid that enables it to pick up the magnetic field transmitted from an induction loop system.  Where installed, induction loops offer the user the chance to hear more clearly, without interference from room acoustics or background noise. Induction loops are common at ticket office windows, in theatres and cinemas and in some shops etc. Look out for this sign, which advertises the presence of a loop system:
In order to use an induction loop (or telephone with an inductive coupler), the user just needs to switch to the 'T' (telecoil) setting on their hearing aids. 
Virtually all NHS supplied hearing aids, and many bought privately, incorporate this valuable feature.
If your hearing aid has this capability, make sure that it has been set up under one of the ‘programmes’, otherwise you might not be able to use it!