Scrip-for-scrip rollover lets you defer paying capital gains tax on rollover equity, the shares you take instead of cash, until you eventually sell those shares.
Technically, it is a CGT deferral provision under Subdivision 124-M of the Income Tax Assessment Act 1997 (Cth) that allows a seller receiving shares in exchange for shares in the target to defer the capital gain on the exchange until the rollover shares are ultimately disposed of.
Eligibility depends on the structure of the acquiring entity, how much of your consideration is actually scrip rather than cash, and a number of other technical requirements that are genuinely fact-specific to the deal in front of you. It is easy to assume rollover equity is automatically tax-effective, given how often it is marketed that way, and find out otherwise after the deal has already been signed.
What you as a seller should ask your tax adviser: does the specific structure of this deal actually satisfy the scrip-for-scrip requirements, before you agree to take rollover equity on the assumption that the tax cost is deferred.