Any system that has dependency between its components can be viewed as a chain. Businesses have components (departments) that are dependent on other components (departments).
The constraint (weak link or department) determines the throughput (strength) of the entire system or business.
Here’s an example: a business may have a lot of sales – it may have the best staff and products in the world – but if it cannot produce them fast enough (not enough production capacity) then the business will not reach its full potential or throughput (it is production constrained).
Often within the entire production process there will be one machine or process that is the constraint.
The same is true in vice versa: a business may be able to make the best product in the world and deliver it anywhere within days and at a great price, but if no one knows about it (lack of sales/marketing capacity) then the business will not maximise its potential (it is sales or market constrained).
Again, within the sales process there will be a specific constraint, whether it be lead generation, sales execution or closing.