SLI is the orientation of two GPUs working together, connected by a small cable that allows them to use the computing power of both cards. Some games do not support this orientation, so it will only use one of your 980s, but it will still work very well. And for those games that do support it, it works like a charm, although some people have encountered some graphics problems once in a while with SLI. We have an SLI information guide that you can check out, it will clear up many of your questions: http://linustechtips.com/main/topic/197327-the-sli-information-guide/