Self-determination theory explains that sometimes we do things because we enjoy them and want to do them. For example, playing a sport you love, going for a hike for enjoyment, and purely doing things for your enjoyment. Those are referred to as intrinsic motivation. Other times we do things for outside reasons. For example, getting rewards, receiving a positive grade, and praise. This is what is known as extrinsic motivation. 

However, over time, we can start to see those extrinsic reasons as intrinsic reason. Like doing homework not just for grades but because we know it helps us learn. According to self-determination theory, this process makes the behavior feel more like our own choice (which is the point of self-determination theory).