ELI5 what is a harness?

EDIT from https://arcprize.org/media/ARC_AGI_3_Technical_Report.pdf:

We seek to fight two forms of overfitting that would muddy public sensefinding:

Task-specific overfitting. This includes any agent that is created with knowledge of public ARC-AGI-3 environments, subsequently being evaluated on the same environments. It could be either directly trained on these environments, or using a harness that is handcrafted or specifically configured by someone with knowledge of the public environments.