https://en.wikipedia.org/wiki/Ventilated_cigarette

Light or "low tar" cigarettes are just regular cigarettes with perforated filter tubes. That dilutes the air, so less cigarette "stuff" is in the air that's inhaled. These perforations pull less air through the lit part of the cigarette, which causes it to burn slower, so it takes longer to smoke. The total dose/exposure is roughly similar but they can claim they're safer.