It's a computer simulation where hundreds of virtual organisms evolve swimming skills. These organisms are called "swimbots". You can set mate preference criteria and thus influence what the swimbots consider as attractive qualities in potential mates. The most attractive swimbots get chosen most often to have little babies, and so their genetic building blocks propogate to future generations. Eventually, swimbots get better at pursuing each other, competing for food, and becoming babes to other swimbots. Local gene pools emerge which compete for sex and food (for energy to have more sex). Eventually a dominant sub-population takes over.
Sometimes, everyone just dies. That's not a bug. That's nature! If you want, you can move swimbots and food around to help your favorite critters survive.
Gene Pool is best appreciated as a virtual Darwinian aquarium in which you initiate a primordial soup, and then occasionally check up on what Virtual Mother Nature is up to - about every fifteen minutes (or every few days, if you are interested in long-term evolutionary experiments!).
Speaking of experiments, In Gene Pool you can "Tweak Ecology", which means that you can change food and energy settings. Question: what happens when swimbots give (ALL) of their energy to their offspring when they mate? Here's a hint: They die. But what does this mean for evolution in the long term? Try it and find out.