The intention of an engagement ring is clear: the wearer has said yes to a proposal of marriage and a wedding date will be set. However, the meaning behind a promise ring depends on the couple involved.
Faithfulness
A promise ring may signify a couple's commitment to stop seeing other people or to remain monogamous.
Purity
For younger couples or those with religious beliefs that people should not have sex before marriage, a promise ring may signify their commitment to abstaining from sex.
Lifelong Commitment
For couples who don't believe in marriage, or those who simply aren't sure if and when they want to get married, a promise ring can declare their lifelong commitment to each other in lieu of an engagement.
Pre-engagement
Especially for younger couples, the promise ring may signify the intention to become formally engaged when they are older. It may be seen as a temporary symbol until the couple is financially able to afford an engagement ring and plan a wedding.