An air-side economizer is a set of controlled outdoor-air dampers that let a rooftop unit or air handler cool a building with cool, dry outside air instead of running the compressor — what the industry calls "free cooling." When outdoor conditions allow, the economizer opens, the compressor backs off, and the building gets its cooling from the weather rather than the electric bill.
Because the energy savings are real, commercial energy codes require economizers on most larger units. Under the 2018 and 2021 IECC (and ASHRAE 90.1), a non-residential fan system of 54,000 BTU/hr or larger — roughly 4.5 tons — must have one, with limited exceptions. Note the threshold moved over time: IECC 2012 triggered at 33,000 BTU/hr, so an older building may not have what a new install now requires. Hot, humid climate zones (such as IECC 1A/1B and ASHRAE 0A/0B/1A/1B) are exempted, because pulling in muggy outdoor air there does more harm than good — the exact exempt list depends on the locally adopted code edition.
For an owner, two things matter. First, on a qualifying new or replacement unit the economizer isn't optional — leaving it off (or installing one that's seized or mis-wired) is a code and energy problem. Second, economizers are a classic neglected-maintenance item: stuck dampers and failed actuators quietly kill the savings, so functional economizer checks belong in any commercial preventive-maintenance scope.