The survival period is how long after completion the buyer still has to bring a claim against the seller for breach of warranty or indemnity. Once it expires, the warranty is dead even if the breach existed all along.
Technically, it is the contractual time limit, measured from completion, within which the buyer must give notice of a claim (and often commence proceedings) under the warranties and indemnities in the share sale or asset sale agreement.
The buyer wants long survival periods because problems in a business often surface years after a sale. The seller wants short survival periods because they want to close the file, distribute proceeds, and get on with life rather than carry tail exposure for years.
Survival is rarely a single number. General business warranties typically get one period, tax warranties and the tax indemnity get a longer one anchored to the ATO’s period of review for the relevant entity (so the seller’s exposure tracks the actual statutory risk), and title, capacity, and authority warranties typically run for the full limitation period under the relevant Australian Limitation Act because they are fundamental to the buyer having received what they paid for.
What you as a seller should push back on: general warranty survival periods that creep towards tax-warranty length, tax warranty survival that exceeds the ATO’s period of review, and any survival period that begins running from a date later than completion (such as discovery of breach or notification).