Database.AllowsCallouts

In plain words: Queueable Apex is an asynchronous execution framework in Salesforce that lets you run resource-heavy or long-running tasks in the background. Unlike legacy @future methods, Queueable Apex allows you to pass complex data types (such as sObjects and collections), returns a trackable AsyncApexJob ID immediately, and supports sequential job chaining to execute multi-step background processes. Synchronous transactions in Salesforce have strict governor limits—such as CPU timeouts and heap size caps—to ensure platform resources remain shared fairly across all tenants. When your business logic requires heavy data processing, complex calculations, external REST callouts, or operations that risk slowing down the user experience, offloading the work asynchronously is critical. Queueable Apex combines the simplicity of future methods with the power of batch classes.