A toroid is a doughnut-shaped object with a coil wound around it that is used as an inductor in electronic devices. The shape is described in mathematics as an object or surface generated by revolving a closed plane around an external axis that is parallel to it so it does not intersect; the resulting shape is doughnut-like, and the referred axis is at the very center of the open space.
Techopedia explains Toroid
A toroid is a common electronic component composed of a hollow circular ring where a number of turns of copper wire are wound. The ring is usually made of powdered iron or ferrites to introduce permeability and increase the generated inductance for a given number of turns of the copper wire.
Toroids generate a magnetic field or frequency depending on the material of the ring (permeability) and the type of wire and the number of turns the wire is wound; the overall size also matters. These pulses are used as a counterbalance in order to reduce noise in an electronic system such as line filters.
Toroidal cores for transformers are gaining popularity compared to straight solenoid cores because magnetic flux leakage is minimal owing to the toroidal shape compared to the latter's straight solenoid shape. The overall functionality is simply greater inductance with minimal electromagnetic interference caused to the surrounding circuits or equipment.