I managed to meet the requirements to feed all prisoners. Much of it is luck, due to the goal requiring you to literally FEED all prisoners, and not all prisoners will be hungry at the same time.
The To-Do list has three icons for the prisoners:
- Red icons for hungry prisoners. If you have a psychologist, I believe this is roughly when their "Food" need is almost full.
- Green Dots indicate prisoners currently eating. This will only happen during Eat activities.
- Green icons are prisoners that are not currently hungry. They will most likely NOT eat during your Eat activity.
Under the prisoners, you see the Meals "bar" (for lack of a better word). That shows how many prisoners you can feed with the food currently on the Serving Tables. Note that that does not take into account food on Cookers or currently in transit to the Serving Tables.
The simplest way to actually feed all prisoners would be to limit the Eat activity to once a day. Doing so, and ensuring you have enough Cooks to make enough food, will ensure you can serve up the maximum amount of that gourmet jail slop.
Note that there isn't actually any reward for being able to feed all your prisoners; it just seems to be an indicator of your prison's current hunger state.