One of the metaphors that they use in Toyota is called the “Lake and the Rocks”. Is goes like this.
The lake is the work in progress (WIP). That could be inventory in manufacturing, it could be partially processed mortgages or patients waiting on trolleys in corridors waiting to be treated or admitted. Anything that represents a partially completed job.
The rocks are the problems that slow the work down or cause errors leading to unsatisfactory work.
The main idea of Lean is to make value flow to the customer as fast as possible. One thing you have to do to make value flow faster, is to remove WIP from the work stream. If you want cars to go faster on a motorway it helps to have fewer cars.
So the lake is the WIP and the rocks are the problems. The metaphor says that as you reduce the the water the rocks will be exposed. So as you make work flow by getting the excess WIP out of the system you will come across problems that will hamper further improvements.
So work on the problems and you can continue the improvement.
The trouble is there are always more rocks. That is why Lean is something that an organisation “is”. Not something it does and then stops.