Amazon's fulfilment service: you send inventory to Amazon's warehouses and they store, pick, pack, ship and handle returns and customer service. FBA earns the Prime badge and a strong Buy Box position - in most categories the Prime badge beats a lower price - but total fees run ~30-35% of the sale (referral, fulfilment, storage, returns processing) against ~15% referral plus your own shipping for FBM. Remember FBA is a fulfilment choice, not an account model: the playbook's recommendation is to sell 3P through Seller Central with Brand Registry from day one, keep price control, then decide FBA versus FBM per SKU.
Benchmark. A common approach is FBA for your top 5-10 high-velocity, standard-size SKUs and FBM for long-tail, oversized or slow-moving inventory. Model the full ~30-35% fee stack before you list - Amazon's P&L looks nothing like your DTC P&L.