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The modal verb ‘may’ can be used to express both possibility and permission.
possibility – something being possible
I may drive to London this weekend.
The doctor may give you a prescription.
I am not sure if I am going to drive; it is possible that I will travel by train; it is possible that I will not go to London at all.
It is possible that the doctor will give you a prescription but it is not certain. They will give you a prescription if you need one.
permission – something being allowed
You may sit on the grass.
You may use your mobile phone in the waiting room.
Sitting on the grass is allowed so you can sit on the grass if you want.
Using a mobile phone in the waiting room is allowed so you can use it if you need to.