What does this mean? 
This happens when you have a slot that manufactures items, but the ‘export’ stockpile for that slot is full. In practice this means that (for example) you are producing seats quicker than you need seats to fit to cars). As a result, the slot has to stop working. In the short-term you can add a ‘buffer’ for this by sticking a supply stockpile nearby that accepts those items only, thus freeing up more space, but in the long run, you will need to have a faster (or additional) production line to ensure you can use those items as quickly as you make them.