When dealing with long-running operations, such as sharing recalculations, Salesforce puts a job in a background queue so that the calculation can be done better using the organization resources. When the job is completed, Salesforce sends a notification to the user who triggered the job, informing them of the completion.

When dealing with large datasets, background jobs can take hours to complete, and we may be asked to monitor its status.

Go to Setup | Environments | Jobs | Background Jobs:

If you need to abort a running job, you need to ask Salesforce Support.