Flat feet appear to have no or very little arch height and excessively roll inwards and down towards the floor when you're standing. You might appear to have a well-defined arch when sitting, however its the position while standing that we're most interested in.
Flat (pronated) feet can place excess strain on muscles, tendons, ligaments, and bones in the feet. This can make you more at risk for injury and pain.
This may be present from a young child or develop gradually as you age.