Joint products are different products of similar values and approximately the same importance manufactured together in the same production process.
An example can be cogeneration unit that produces both heat and electricity.
As the production or its part is common, joint (common) costs are incurred and they need to be split them between the products. It is usually done by allocating them using a reasonable allocation basis (sales value, weight…). More here.
And what are the indicators to distinguish joint products from main and by product? Read about it here.