A holdback is part of the purchase price the buyer keeps for themselves after completion, rather than paying it to you, to cover potential claims or adjustments.
Technically, it is a portion of the price retained directly by the buyer, rather than placed with an independent third party, released to the seller once the relevant period or condition has passed.
The buyer wants a holdback because it is simple and keeps them in control of the money. The seller should generally prefer escrow instead, because a holdback leaves the buyer holding both the decision about whether a claim is valid and the funds to pay it.
The practical risk is release. Nothing forces a buyer to pay out a holdback on time if they can find a reason, or a dispute, to delay it. Money you are owed but cannot compel payment of is not meaningfully different from money you do not have.
What you as a seller should push back on: any holdback where escrow was a realistic alternative. Where a holdback cannot be avoided, insist on an automatic release date and interest on amounts not released on time.