Pre-existing conditions are medical conditions (problems) that exist at the time you apply for medical insurance. A few examples are shown below:
Surgery, while going through the healing process, can be considered a pre-existing condition. The healing process can normally be up to five years. This can also be true for fully removed cancer. But this may not be true for heart surgery; no matter how successful the surgery is, a heart surgery can weaken the heart a bit.
A hernia operation 30 years ago was recently declared to be a pre-existing condition by a client's treating physician; apparently, the original surgery never fully healed properly. The insurer, therefore, declined to pay for the new surgery.
Conditions like Diabetes and Hypertension, that can be controlled but not cured, are always considered "pre-existing conditions" no matter how well they are controlled.