subject + have/get + something (object) + past participle
Used to talk about arranging for someone else to do a particular task or service for you.
The subject of the sentence is the person who wants the task to be completed, and the object is the task itself.
You are not doing the task yourself; instead, you are arranging or hiring someone else to do it on your behalf.
For example:
I had my car repaired – This means you arranged for someone else, like a mechanic, to repair your car.
She had her hair cut – This means she went to a hairdresser and asked them to cut her hair.
They had their house painted – This means they hired painters to paint their house.
Mary will have her nails done – This means that Mary will go to a nails salon and the nail artist will paint/do her nails.