Short definition
The map pack is the box of three local businesses with a map that Google shows at the top of geo-intent search results — the most clicked feature on a local SERP.
In depth
Owning a map pack slot for high-intent terms like 'plumber near me' or 'best HVAC [city]' is worth more than the #1 organic position for many service businesses, because the map pack appears above the fold and is the default place mobile users tap. Ranking factors are dominated by distance from the searcher, GBP completeness and category accuracy, review volume and recency, and the consistency of NAP across the web.
Example
A roofer holding 1 of 3 map-pack slots for 'roofing [city]' generates 38 inbound calls a month — more than the top three organic results combined.