"Claims Made" basis means it covers you for claims made against you and which you notified to your insurer during the Period of Insurance. The actual mistake causing the claim can occur at any time in the past (as long as it is later than the retroactive date). This is an important feature because in providing professional services/advice, e.g. the result of a fault in the design of a bridge may not become known until quite some time after the design was made.
On the contrary, "Occurrence" basis, which applies to most properties insurance, will cover you for claims arising from events that occurred during the Period of Insurance. Usually the insurer would like to be notified as soon as possible but there is no requirement for you to notify them within the Period of Insurance.
