"Break down" has 8 definitions.
In general, "break down" means "to stop working" or "to remove obstacles." This can be applied to a variety of different situations.
1. When a machine or vehicle stops working.
"If the car breaks down again, I won't have enough money to get it fixed."
2. When communication or a relationship has failed because of a problem or disagreement.
"The discussion was going so well until it finally broke down."
"Their relationship eventually broke down because he was addicted to gambling."
"Break up" or "Break down"
A similar phrasal verb can be used, "to break up," to mean they separated.
"They eventually broke up because he was addicted to gambling."
3. A sudden collapse in one's mental or physical health.
"His health broke down due to overworking."
It is also quite common to use this in the noun form.
"He had a nervous breakdown."
4. To start crying or become very upset.
"After hearing the bad news, he broke down and cried."
5. To break a door or a wall.
"The firefighters broke down the door to get inside the burning building."
6. To remove a difficulty or an obstacle that is preventing something from happening.
"There may be some barriers that need to be broken down for a smooth transition."
7. To divide something into separate parts.
"Breaking it down into smaller tasks makes it more manageable."
This can also be applied to chemical substances when it is undergoing decomposition.
"A chemical compound is broken down into two or more elemental parts or to simpler compounds."
8. To explain something step by step.
"I'll break down the lecture for you."
This can also be applied to numerical analysis. However, these are usually specific business terms in the noun form.
"A cost breakdown analysis is required for each step of the project."