What is Ad set?

An ad set is the middle level of a Meta ad campaign, between the campaign and the ads. The campaign sets the objective, and the ads hold the creative. The ad set holds the settings shared by every ad inside it: the audience, placements, schedule, bidding and optimisation goal, plus the budget unless the campaign uses Advantage+ campaign budget.

Meta's delivery system learns at ad set level. Each ad set goes through its own learning phase, which usually ends after about 50 results in the week after its last significant edit. Meta's developer documentation suggests an ad set for each target audience, while its help centre recommends combining similar ad sets so each one gets enough results.