When you apply for life insurance, you will need to give details of your medical history so that the insurer can assess how likely it is that you will die during the term of the policy, and by extension the probability that it will have to pay out.
If you have had health problems in the past, or if you want to insure for a particularly large amount (what is considered a large amount varies from insurer to insurer), your insurer is likely to ask you to provide a medical report from your GP – and/or you may be asked to attend a medical examination by a doctor chosen by the insurer.