This is one of the best theories I've read. This was formulated by BJ Fogg. The idea is that behaviour is a function of three things -
M - Motivation. How motivated is a person to do this? It can be external motivation such as salary, bragging rights, or internal motivation such as taking care of health, being of good integrity.
A - Ability. How easy it is for the person to achieve it. The more the friction, the tougher it is for the person to exhibit that behaviour.
P - Prompt. Does the person know about or get nudged at the right moment to do this task/behaviour?

Take, for example, going to the gym:
Motivation - I think the biggest problem is motivation. No one is paying you money to do that. But actors, models, and sportspeople whose livelihood depends on their body go to the gym regularly because that is what pays their bills. Physical fitness is not the only factor but an important one. But normal folks need a physical trainer or health risk alert to get to the gym, and even then it is not easy a lot of the days.

Ability - If you have to go to the gym that is far away and it is raining outside, that reduces the chance of you going to the gym regularly. But if you had it within your apartment premises, that makes it a little easier. If you had it at your home - even easier.

Prompt - Do you have a time or alarm to get to the gym?

Now you will observe that you might have any two of those but still you won't go to the gym. You need all three to be able to go to the gym. That is what B=MAP is all about.

This can be applied to get something done by your kids, by you, or by your reportee in the office.