Chastity (or purity) rings serve as an outward sign both to the person wearing it and anyone who sees it, that the one wearing it is committed to chastity. Everyone who has been baptized is called to chastity, in imitation of Christ (CCC 2348), though not everyone needs to wear a chastity ring to signify this.
In general to be chaste means to successfully integrate your sexuality within yourself so that you are able to be spiritually and bodily united (CCC 2337). This means that you not saying things with your body that do not represent what is true spiritually.
For example, it would be unchaste to have sex before you are married, because you are saying something with your body – “I give myself completely to you and you only for the rest of my life” that is not true.
It’s important to see that what chastity means practically differs depending on your state in life. For example, if you are unmarried, it includes abstinence; if you married it does not.
This promise – to only say what you mean with your body – is what chastity rings signify. Normally they are worn by those who plan on getting married and are currently striving to live a chaste single life.
Chastity rings are traditionally worn on the ring finger of the left hand, holding the place for the wedding ring they will receive from their future spouse. They serve as visible reminder for the person wearing it that they are going to remain abstinent until their wedding night, when their heart and body will be saying the same thing.
In practicing chastity, we are imitating Christ, who practiced it perfectly. Those who wear a chastity ring can imagine that Christ is holding their ringed hand until He gives that hand to another in marriage.