In a perfect world, your company would continually have enough inventory on hand to fill all of your customer orders and ship them on time. Unfortunately, this isn’t always the case in the real world.
It could be that one of your suppliers of raw materials is having problems shipping you what you need. Maybe a machine on your shop floor is down for repairs affecting your capacity to produce. Or, maybe your ERP software doesn’t give you the ability to properly forecast what you need to keep in stock, both saleable items and the components needed to produce those items.