If a second verb follows the verb "poder" (be able to), the verb which follows "poder" is used in the infinitive. For example: "Yo puedo levantar 100 kilos." (I can lift 100 kilos.) You will notice that the verb that follows "puedo" is in the infinitive. So "puedes darme (algo)" means "You can give me (something)." If you are asking a question, ¿puedes darme (algo)?" means "Can you give me (something)?" Therefore, ¿Puedes darme el plato de arroz, por favor?" means "Can you give me the plate of rice, please?"
"Dame" is a command; an order. So "Dame el plato de arroz, por favor" means "Give me the plate of rice, please."