Blockchain scenarios are interactions with the blockchain, real or imagined.
They help developers write tests and are able to document and replay interactions with smart contracts.
The format is described here: https://docs.multiversx.com/developers/scenario-reference/overview
This Go framework deals with reading, writing, and controlling scenario runners.
Scenario runners are the routines that do something with these scenarios. Think of them as closures that receive the scenario steps.
The main example for such a runner can be found in the VM, here: https://github.com/multiversx/mx-chain-vm-go/tree/master/scenarioexec
However, more such runners are conceivable.
To implement such a runner, create an object that implements interface ScenarioRunner.
There is an equivalent Rust implementation here: https://github.com/multiversx/mx-sdk-rs/tree/master/sdk/scenario-format
The Go implementation (this one) is older and generally tends to be better featured, altough they should be up-to-date with one another now.