An accessibility champion is typically not a full-time accessibility specialist but a team member, a designer, developer, or product manager, who takes on extra responsibility for keeping accessibility visible in day-to-day decisions: flagging issues in design review, answering basic questions, and escalating harder problems to a central accessibility team or outside consultant.
Organizations that scale accessibility successfully across many teams almost always rely on a network of champions like this, since a small central accessibility team cannot realistically review every design and every pull request across a large organization by itself.