Boosting is a deliberate adjustment to a result's score that promotes or demotes it for business reasons, independent of its textual relevance.
Common boosts include in-stock items, bestsellers, new arrivals, high-margin lines, promoted collections and seasonal ranges. Negative boosts handle clearance, discontinued lines and low-rated items.
Boosts are multiplicative pressure on an existing order, and they compound badly. Applied heavily enough that they overrule intent, they produce the widely disliked experience of searching for one thing and being shown whatever the site is pushing this week.
The safe design rule is that boosting should break ties among comparably relevant results, never rescue irrelevant ones.