As the name implies, a promise ring represents a promise and a symbol of commitment between a couple. Usually this promise is related to the relationship. It's a way of showing devotion and openly declaring your love and dedication to the relationship.
Sometimes, a promise ring is referred to as a pre-engagement ring. For couples that don't have plans for marriage yet, a promise ring can be a symbol of their intention to one day get married.
If there is some obstacle to marriage, like geographic distance or a financial obstacle (or even age if a couple is very young), a promise ring can show a couple's commitment and act as a placeholder for an engagement ring. Alternatively, if a couple never plans to marry but wants to commit to each other, they might use the term "promise rings" instead of wedding bands.
The beauty of a promise ring is that a couple's promise to each other is as unique as they are. A promise ring can symbolize whatever the couple chooses, so that it's meaningful and personal to them. It can even symbolize a promise between friends who are platonic.