Fischer projection of monosaccharides is a tool to draw the monosaccharides in none cyclic ring and open-chain structures showing the chiral carbon atoms and the different isomers of the monosaccharides depending on the location of last hydroxyl group being at the left of the Fischer projection as L or being at the right of the Fischer projection as D.
The aldehyde carbonyl group is always at the top of the molecule as the first atom of the monosaccharide. The ketone carbonyl group is always at the second position of Fischer projection.
Monosaccharides Fischer projection exhibit several chiral carbon atoms.
The videos illustrate the topic of Fischer projection in molecules.
https://www.youtube.com/watch?v=-V0e9ex8iHQ
https://www.youtube.com/watch?v=-V0e9ex8iHQ
Examples:
Starting with a wedge-dash Structure Ending with Fischer structure
Note that carbons atoms 1, 2, and 3 are chiral carbon atoms. Also, note the carbonyl aldehyde always at top of Fischer structure and the carbonyl ketone is the second position at the Fischer structure.