In many cultures and restaurants, putting your napkin on your plate after finishing a meal is a signal to the staff that you have completed your meal and that you are ready for them to clear the table. It's a polite and common way to indicate that you are finished eating and that the staff can start the process of cleaning up the table for the next guests.
By placing your napkin on your plate, you are signaling that you are no longer going to use it and that it can be taken away along with any other dishes or utensils that may still be on the table. This action helps the restaurant staff efficiently manage table turnover and provide good service to other customers waiting to be seated.
However, it's important to note that customs and signals can vary from one culture to another or even from one restaurant to another. In some places or higher-end restaurants, you may be expected to place your napkin neatly beside your plate or on the table to the left of your plate, instead of directly on the plate. In any case, the restaurant staff is usually attentive and observant, so they will understand the message you are trying to convey with your napkin placement.