What do the little squares of the graphic, represent in the structure of a partition ?

All is in the title

depending on screen resolution, hard drive size and how much screen space you have allowed DF to use in showing the graphical map of the drive, each block represents a 'portion' of the hard drive where your files live.

so some blocks may contain 1 file, or maybe 100's of files, it's highly variable.

when defragging, you sometimes see white (empty) blocks and blue (no fragments) being played with due to 'rounding'.

you can click on any of the blocks to see what it contains.