Credit card churning is the practice of opening and closing credit card accounts in order to get sign-up bonuses and credit card rewards, such as cash back or miles. Credit card churners often sign up for multiple cards at a time, keep them open long enough to get the introductory bonus rewards, and then cancel the cards (ideally before incurring any fees).
While credit card churning might seem like a good way to get more rewards, you risk hurting your credit score and may rack up avoidable debt.