A managed network hub that transitively interconnects multiple VPCs and on-premises connections (VPN, Direct Connect) through a single central point, avoiding the complex mesh of point-to-point peering.
In SAP-C02 the key difference from VPC Peering is transitivity: with TGW, VPC A can reach VPC C through the hub without direct peering; VPC Peering is never transitive. It can be shared across accounts in the organization via AWS RAM.
AWS Transit Gateway (TGW) is a concept that can show up on the exam. Check the full guide to see it in context.