What is a workplace pension?
A workplace pension is a retirement scheme arranged by an employer. Under automatic enrolment most eligible UK employees are placed into one by default, with contributions from both the worker and the employer. Schemes are either defined benefit or defined contribution.
Opting out means giving up the employer contribution, which is why it is almost always the wrong decision. Contributions attract tax relief, either through net pay or relief at source depending on how the scheme is run.
The structural problem with workplace pensions is job mobility. Each employment leaves a pot behind, and after several roles most people have several schemes, often with no idea of the values, the providers, or where the paperwork went. The Pension Tracing Service exists precisely because of this.
Two things go stale silently: the address the scheme holds, and the death benefit nomination. Both matter more than the fund choice, and neither prompts you to review it.