Self-monitoring or tracking simply refers to a person measuring their behavior over time.
Often, merely tracking a behavior can influence the likelihood or frequency with which a person performs the behavior or related ones. For example, many pedometer studies increase walking activity merely by improving awareness, and many interventions that merely consist of rewarding someone for weighing themselves result in weight loss.
That said, people often find tracking behaviors tedious and lose interest after a short period, so behavior designers should seek to reduce the burden of self-monitoring by collecting information automatically or doing so in a low-cost way.