TL;DR
A migration bundle packages every pending migration for one DbContext into a single, runtime-targeted executable. No source code or dotnet-ef tool needed on the target machine.
They exist to get schema changes out of your running application and into a single, deliberate, auditable step, which is what makes them safe for production, CI/CD, and multi-service/shared-database setups.
They're idempotent, one-per-context, and need an appsettings.json sitting next to them with the right keys present (not necessarily real values).
The tool sometimes logs in red when there's nothing to do. That's not a failure, check the last line.
