When pouring concrete, one of the most important considerations is the workability of the concrete. In other words, how well does the concrete flow.
If the concrete flows well, then it is easy to ensure that the concrete will fill the whole volume of the formwork without leaving any small air pockets. To quantify this flowability of the concrete, it is a very common practice to use a slump test.
Slump Test Setup
The slump test is performed using a small metal bucket with fresh concrete, as shown in the image below.
After carefully removing the bucket, the concrete will "slump" under it's own self-weight. The amount which the top of the concrete cone drops is known as the slump.
Significance of Slump
As mentioned above, the concrete's slump value is an indication of the concrete's workability.
General Requirements
Rather than setting a minimum slump value, it is instead common practice to set a maximum. In Japanese building design practice, the maximum concrete slump is typically 180 mm for most common grades of concrete (below 50 MPa cylinder strength).
The reason for setting a maximum limit is that there is a trade off between concrete strength and workability. Once you exceed the maximum, the concrete strength is adversely affected.