A rider is an optional add-on you can attach to your base insurance policy, usually for a small additional premium, that extends your coverage to specific risks not included in the standard policy. Common riders on life insurance include critical illness cover, accidental death benefit, and waiver of premium (which waives future premiums if you're disabled).
Riders let you customise a policy to your specific needs without having to buy an entirely separate policy for each additional risk, and they're often more cost-effective than standalone coverage for the same benefit. That said, it's worth evaluating each rider on its own merit — not every rider is necessary for every individual, and sometimes a separate, dedicated policy (like standalone critical illness insurance) may offer more comprehensive coverage for a similar cost.