Outbound network traffic leaving your cloud environment to the internet or other networks. Like mail going out from your office - cloud providers often charge for data leaving their network.
When your application sends API responses to users or backs up data to another provider, that's egress traffic which may incur bandwidth charges.
All major clouds meter and often charge for outbound data leaving their networks (to the public internet, other clouds, or external networks). Names differ, but the concept is the same: bytes sent out are egress and may incur per-GB charges depending on destination, region, and service.
Explore real-world architectures from our community that use Egress: