1. to become emotional or start crying
broke down in tears
The phrasal verb 'break down' can mean a mechanical or technical failure, such as that of a vehicle, or to say that something or somebody is not functioning normally. If a car breaks down, it literally stops working. In a figurative sense, that meaning can also be used to refer to the failure of a plan, relationship, negotiation, or even the human body. If that has stopped functioning as it should or is in bad condition, you can say that it has 'broken down.' The same can be said if your mental health is affected by something negative, like stress or sadness. In that case, you can also say that you have 'had a breakdown'. Similarly, to 'break down in tears' is to be overcome by emotion so that you lose control. This phrasal verb can also be used in reference to separating something into different or smaller parts. You can 'break down' something by explaining it step-by-step, analyzing it, or arranging it into different categories. Similarly, if the structure of something separates or decomposes, you can say that it 'breaks down.'
2. cease to function or operate properly
the car/bus/train broke down
If an object 'breaks down' it stops working or develops a fault. This is used often with vehicles or machinery but can also be figurative such as a 'break down in talks' or a 'break down in a relationship'. Used in professional and social contexts.