Changes can break old behaviour
An online shop adds a discount, but then refunds start giving the wrong amount. That is a regression: something that used to work has broken after a change.
Regression testing repeats checks of existing features after changes. Automated checks can run often; people can also retry important tasks that need human judgement.
Choose checks by risk
Teams run quick checks with each change and broader checks before release. They focus on parts where a failure would hurt most. Passing tests cover only the situations they check, so when a bug slips through, teams can add a check that would catch it next time.