This is insurance that pays out when a defined medical event occurs.
For example, following a heart attack, stroke, cancer or some other specifically defined critical illness.
Cover is for a set term, which may be equal to a mortgage term, for when children have grown up, until retirement or another life stage milestone. It may be worth considering having one policy for a set term to cover the mortgage, and another that will provide money to help provide for your different lifestyle if a serious illness happens.
Most people choose a lump sum to be paid out. There is the option of receiving it as set income over the term remaining, which is often a lower cost option.