The retroactive date is the date from which claims for work performed will be covered, which is normally the date you started buying insurance.
It’s important, therefore, to take out E&O insurance as soon as you start trading, although some policies allow you to backdate your cover date for an extra fee, and others may state ‘retroactive date - none’, which would cover all past work.
Your policy may be invalid if you backdate cover in the knowledge that a claim from that period is likely to be made.