A treat meal may be a candy bar or a slice of pizza or something that meets your calorie requirement for the meal but is not exactly healthy.
That means on 4 days, you may consume something that you crave as a treat meal. However, it must be of a portion that does not put you in a caloric surplus. So, you will still be in a deficit but you would have gotten to enjoy eating something you really want.
There are 2 reasons for treat meal. A strict diet can be extremely stressful mentally. You are in a constant state of denial. Your body will crave unhealthy foods such cakes, ice cream, etc. It takes discipline and determination to avoid these temptations. Over time, this will take a toll on you mentally.
By having a treat meal, you satisfy your craving and get a psychological boost. You get the motivation to keep going.
Another reason would be that these treat meals may give your metabolism a boost. If you are eating steamed chicken breast and broccoli daily, a slice of pizza may be a surprise to your body. It is a high GI food and this will spike your insulin and blood sugar levels causing a rise in your metabolic rate. Since it is just a small meal, it will not do any real damage and you will not pile on the pounds.
What really matters is how compliant you are with your training and nutrition. Most people do not see results because they are only about 20 or 30 percent compliant. If you are supposed to run 5 times a week and you only run once, that's a 20 percent compliancy. It will take you ages to see any real significant weight loss.
If you go for all 5 sessions, that is excellent. You have 100 percent compliancy and you should be proud of yourself. Keep it up and you will reach your weight loss goals in no time.
Treat meals are not to be mistaken for cheat days. On a cheat day, you may eat anything you want for the entire day. You are supposed to go on a calorie surplus by about 500 to 800 calories. This is meant to shock your system and boost your metabolic rate.
When you are on a calorie deficit diet for 6 days, your body's metabolism will naturally start slowing down. A cheat day on the 7th day will kick start the fat burning process by giving a major boost to your metabolic rate. So, note the difference.
Treat meals are not meant to create a caloric surplus. They are just there to ease your mental stress and motivate you to be more compliant.