These are the roller wheels located at the bottom of an inline sliding patio doors.
Roller wheels on aluminium patio doors are usually fastened in from under the glass so the bottom section of the patio frame needs to be removed to get to the fixing screws.
Two screws hold the bottom section of frame in place. They are located on each of the side sections of frame through a hole near the bottom of the door.
The door must be lifted out of the frame and laid flat before this work is undertaken.