A ring is a circular object (a bit like a round band of matter), with a void center.
The word is used in many contexts. It is a precious object when it is set of jewels (like a ruby or a diamond), made of a precious metal (like pure gold!) and that is fits perfectly the finger of a lady (or to some extent of a man).
In this case, the ring can either be an engagement ring or a wedding ring.
The engagement ring is usually offered by a man who asks for her future fiancé's hand in marriage.
It is supposed to value the love of the future groom who kneels to put the ring on her fiancé's finger.
If the lady agrees on keeping the ring, the two fiancés are bound by a non-official contract that must conclude on a wedding.

The two wedding rings (one for each spouse), in a religious ceremony, is exchanged in front of a priest. It symbolizes the union of the couple that gets married. The ring indicates commitment, and epitomizes one of the marriage vows: fidelity.

A ring is not always a precious item. It may be anything that is circle-shaped and sort of hollow in the middle.
In sports, a ring is the delineated zone where matches occur.
A playing field, in combat sports like boxing or wrestling, is a raised square structure. There is a post at each corner of the boxing space, from which four parallel ropes are attached, allowing the boxers to combat without taking the risk of falling from the platform.
Two corners are reserved for each of the fighters

but a ring may be many more things: a round motorway for vehicles to drive around a city, a gastric ring (a ring that tightens the stomach of obese persons, used to simulate satiety).
It is also the name of a Japanese horror movie where people die after watching a “cursed” videotape in which a series of weird images appear. The ring is one of the sequence in the video and actually represents a well seen from above.