Cuckolding is a sexual practice or fantasy in which a male obtains sexual pleasure from seeing his female partner having sex with somebody else, typically with someone who is more physically attractive than himself. Sometimes the man might also exist while his partner is having sex with someone else, but he is not involved in the act himself.
The word cuckold originates from the Old French word cocu, which suggests "cuckoo bird". The cuckoo bird lays its eggs in other birds' nests and the unsuspecting host birds raise the cuckoo chicks as their own. A guy who is cuckolded is the unwitting "host" of his partner's infidelity.
People might get into chastity cuckolding for a range of factors. https://thumbpower.org/?p=671 For some, it is a way to add excitement to their sex life.
Whatever the factor, people who have an interest in chastity cuckolding ought to guarantee that all celebrations involved are consenting which everyone is comfy with the plan. It is also important to set clear borders and expectations before embarking on this type of sexual adventure.