NEAT (non-exercise activity thermogenesis) is the energy you burn through movement outside of planned training: walking, taking the stairs, standing, housework, your job and general fidgeting. Together with your basal metabolic rate, training and digestion, it makes up your total daily energy expenditure (TDEE).
Why NEAT matters so much
How much you move during the day varies a lot between people: someone who sits at a desk and commutes by car burns far less than someone with a physical job and lots of steps. Calculators squeeze all of that into a single activity factor. Just choosing between "moderately active" and "very active" can shift the result by 300 kcal or more.
NEAT during a cut
As your weight drops, so does your energy expenditure, and many people unconsciously move less during the day. Trexler, Smith-Ryan and Norton (2014) describe this adaptation in their review. Your deficit shrinks over the weeks even though you eat the same. So keep an eye on your everyday movement, for example through your step count.
The flip side: more everyday movement is a simple lever. Extra steps increase your deficit without putting much strain on your recovery from lifting.
NEAT during a bulk
When building, your maintenance needs rise with your weight and often with more everyday movement too. What fit in the first month may be too little by the fourth.
You do not need to measure NEAT. Any change is automatically reflected in your weight trend: if the trend falls behind your plan despite the same calories, a change in everyday movement can be one reason.