That is your progress score, basically a rating of how well Memorang thinks you know a topic. As you answer correctly, the score will go up, and if you struggle on a topic, scores will go down. Every score starts at 50%, a neutral value, since Memorang doesn't know your current knowledge level and starting at 0% is pretty pessimistic! Once your scores get into the green range around 80% it means you know the topic fairly well. Getting to 100% is much harder as it entails totally mastering the topic beyond a doubt.