A 'food tray is an object used as a dish in Prison Architect.
When the eat regime is active (or just before), a cook will put clean food trays on a serving table in a canteen, along with food. Prisoners will take one food tray, along with cooked ingredients. When they have finished, they will leave a dirty food tray, which will be washed in a sink by a cook to reuse.
Food trays cannot be bought or sold, only dumped. The amount of food trays is controlled automatically: if there is not enough of them, more will be bought. Sometimes their amount goes beyond prisoner capacity.