Airlines hold an enormous amount of customer data and rarely see it in one place. Bookings sit in the reservation system, loyalty in another platform, complaints in a service desk, corporate contracts with the sales team and disruption communications somewhere else entirely.
A CRM (customer relationship management system) brings those threads together into a single view of the passenger, the travel agency and the corporate account. When someone gets in touch, the person handling it can see the full history rather than the fragment their system happens to hold.
It doesn't replace your reservation or departure control systems. It sits alongside them, holding the relationship: preferences, entitlements, past disruption, open cases and commercial agreements.
For most airlines, a CRM is where the following live: