There are no real set rules for mealtimes. Everyone is different and everyone has different needs and preferences, especially when it comes to eating and their mealtimes.
The usual procedure is to eat food at three different times in a day. More often than not, these three mealtimes will consist of eating in the morning (breakfast), in the afternoon (lunch) and in the evening (dinner).
At these intervals, you provide your body with enough calories and energy to perform all processes and functions that it needs until the next time you eat.
Sometimes though, we feel hungry between these mealtimes. In these periods of hunger, we may feel the need to eat more food. Snacking in between our main meals of the day can help us to stave off hunger but also cause us to take in too many calories and gain weight. If we choose to eat healthy snacks, however, then this can actually help us to reduce the need to eat between meals and allow us to maintain a higher metabolic rate which means we can constantly burn calories and body at a higher rate.
You should generally try to avoid eating food after dinner. This means that having had a meal in the evening, it is not such a good idea to eat anything afterwards as eating prior to going to bed will prevent you from sleeping properly. The food tends to remain in the stomach during the night undigested and can lead to indigestion.
By staying focussed on a strict eating plan of three well balanced and nutritious main meals each day and if necessary some healthy snacks in between, you provide your body with all the nutrients it requires as well as all the energy you need for the tasks you embark upon. If you choose to eat more than this, you'll probably need to exercise that much more in order to burn off sufficient quantities of calories and body fat to ensure you maintain your target weight. If you do not, you'll gain weight and likewise, if you eat less food than you require to supplement your energy requirements, then you'll probably lose too much weight! The healthiest way to live is to eat sensibly and exercise regularly.