Things don't always go right in Factorio. Sometimes your assemblers or chemical plants may get bottlenecked, which will result in halted production.
This useful mod puts tiny light indicators on your buildings that show three different colors: green, yellow, and red. Green means that everything is alright. But if you see yellow or red, it means that there is something wrong either with your input or output -- so you can respond accordingly.
These simple lights will help you decide which of your production units require immediate maintenance.