Q:

What are these markings under the passenger door?

Is there anybody who can explain what the markings just below the passenger door edge do?

A:

They appear to be alignment marks for the passenger loading bridge (PLB) and air stairs. Some airlines choose mark their aircraft and bridges, in order to prevent damage to the aircraft door due to misalignment of the bridge or air stairs.
On the PLB, there will be a corresponding mark on the floor, at the edge.
The two lower marks would be used for air stair alignment (i.e., when a PLB is not available).
Note the marks at the edge of the bridge in this photo (the position of the alignment marks are not standardized and are carrier specific):

