A waiting period is a specified duration after you first buy a health insurance policy, during which certain claims aren't covered by the insurer. This typically includes an initial waiting period (often 30 days) for most illnesses, and a longer, separate waiting period (commonly 2 to 4 years) specifically for pre-existing diseases you had before buying the policy.
Different insurers and policies have different waiting period durations, and some policies offer to reduce this waiting period for an additional premium. Understanding your policy's specific waiting periods is important, since a claim made during a waiting period for a covered condition can be rejected, which is why it's worth buying adequate health insurance well before you anticipate needing it, rather than waiting until a health issue is already on the horizon.