Conceptual design is the primary work that most designers do at the early stage. It is putting some ideas on the drawing board. At this stage, the designer defines the general lines of the design. The designer decides the strategies to create the design.
This is the stage to know about the processes to take the design through. A common design in the early stage has many sketches with different ideas.
We can say that conceptual design gives us an idea about the problem a product is going to solve. It also reveals how the product will solve the problem and gives us a final outcome.
So, this is the stage when a design is being imagined. The designer is examining all the probabilities to give the design a different and innovative look.