Question about gel electrophoresis which is used to measure the concentration of DNA?

There is a total of 8 wells and the the two ends of the wells are not used, meaning that only 6 wells are used in this experiment. The first four wells are filled with markers while the next two wells are are filled with DNA samples. Below are the amount of markers and DNA which are filled in their respective wells (in microL):

M1 : 1

M2 : 3

M3 : 5

M4 : 10

DNA 1 : 3

DNA 2 : 5

This procedure is carried out to measure the concentration of DNA. Now, what I don't understand:

1. what is the function of the markers

2. why are four markers needed

3. how do you know how many microlitre of markers needed in each well (why 1, 3, 5 and 10)

4. why do you need two DNA samples when they're the same DNA, only different amount

5. how do you know how many microlitre of DNA to put in each well (why 3 and 5)