The "soft swing" term was invented and first used by Club WideWorld of Southern California back in the 70s to describe the club's philosophy. Soft swinging generally refers to people who don't actually swing but enjoy having sex in the same room with another couple. The original meaning referred to people who were more into swinging as a social experience rather than as a hard core pursuit, but over time, them meaning has changed. In general, it typically means that you do not have intercourse with partners other than your own, but some may allow oral or just the girls playing together.
As to how to keep your wife interested, the best thing is through learning more and discussing it. The easiest way to get started is by going to a swinger's dance social. They have these all over the world, and no sex takes place at the dance (usually referred to as an off-premise event). Its a chance to be in a sexy environment without taking it further than you are ready for, and a great way to meet other swingers and discover that they are mostly pretty normal.