Negative Adjustments

When you have a device synced to your MFP account, it works together with the information you have given MFP including the calorie goal you have, as a result of your chosen activity level.

For example:

I am set at lightly active which means MFP expects me to hit around 5000 steps from my device, my calorie goal was increased when I changed from Sedentary (1.25 x BMR) to Lightly Active (1.4 X BMR) to account for this.

If I don't hit 5000 steps over the day then I am not hitting my lightly active NEAT (Non-Exercise calorie burn), with negative adjustment enabled this allows MFP to reduce the calorie allowance to take into account not being as active as intended. So if I only hit 3000 steps for the day then I will have a minus figure taken from my exercise and will have less calories to consume for the day.

TL;DR if you don't consistently hit your activity level every day the negative adjustment reduces the risk of over-eating by adjusting your calorie allowance down.