Overview
minecraft:npc is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Turns the entity into an NPC that creators can give dialogue and buttons in-game, like the vanilla NPC used in adventure maps.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:npc": {}
}