Calling a woman seeking out younger men for sex a cougar initially referred to an older woman who hangs out at bars and pounces on any available and willing younger man at closing time. This references an actual cougar's opportunistic hunting practice of targeting young or weak prey.
While initially a rather negative term, it now has a more positive connotation. It's seen as referring to women in their 30s or 40s who are much more stable, mentally and financially, than women were expected to be in years past. Therefore, these women are secure enough in both their own self worth and sexuality to pursue whom they choose for sexual partnership, even with an age gap of 10 years or so.