Best Match considers the Score and Boost values for products to appropriately display results in an order that accounts for the relevancy of an items and any boost or bury rules that have been applied to the rule.
The score value is based on the fields that have been labeled as "queryable" and returns a number that reflects how well the item matched the searched term in these fields.
The Boost value is based on Boost Rules that have been configured within the dashboard. When multiple rules are affecting the same item, the boost/bury scores will be averaged.