Seat cover side airbags are specially made seat covers that take into account the fact that many seats have side airbags built into them. Until quite recently, it seemed that many aftermarket seat cover manufacturers forgot this small piece of information and the result was not good. The seat covers never deployed the way they should and left a mess underneath what were, sometimes, very expensive seat covers. Today, though, manufacturers realize there are side airbags routinely included in vehicles, and they now make seat covers with patches that break away so that the side airbags can be used safely.
For more info, see The Dangers of Car Airbags