The basic rule
Sanctioning bodies typically require a champion to defend their belt periodically against whichever fighter that body ranks as the top available contender, known as the mandatory challenger. The rule exists to protect deserving fighters from being permanently avoided by a champion who would rather fight easier, more profitable opponents, and in principle it keeps the ranking system meaningful by forcing the champion to prove their status against the person ranked to deserve a shot.
Why champions sometimes resist
A mandatory challenger is not always the most commercially attractive opponent available, and champions and their promoters can have strong financial reasons to prefer a different fight, so the tension between the rule's intent and its incentives is constant and well understood inside the sport.
The purse bid workaround
Most bodies allow a champion to postpone a mandatory defence by triggering a purse bid or paying a step-aside fee to the challenger, effectively buying time to take a more lucrative fight first. This workaround keeps the system flexible but also means mandatory status guarantees a fighter money or leverage more reliably than it guarantees them an actual fight on any particular date.