It is an insurance plan which has fixed cash benefits for medical expenses and no deductible payments. Examples of such benefits are critical illness plans, accident cover plans amongst others. As the name suggests, a lump sum cash amount is paid out to the policy holder in case a legitimate claim is made.
Typically used in the context of health insurance benefit provided by the employer of the policy holder. Flexible coverage implies that employees can pick and choose particular benefits in a plan/package based on their own assessment and needs.