Slump is how many inches a cone of fresh concrete settles once the mold is lifted away, and it's the quick field check of how wet and workable a load is.
The test is ASTM C143. You fill a 12-inch-tall cone (8 inches across the bottom, 4 across the top) in three layers, rod each layer 25 times, strike it off, and lift the cone straight up in about 5 seconds. Then you measure how far the top dropped, to the nearest quarter inch. Do the whole thing in about two and a half minutes, on a proper sample from the load.
Slump tells you about consistency. Stiff concrete is hard to place and finish. Soupy concrete finishes easily and ends up weaker, shrinks more and cracks more. The spec or your mix order sets the target slump.
The trap is water. When a load shows up stiff, adding water is the fast fix, and it costs strength. NRMCA's rule of thumb is about 1 gallon of water per cubic yard for each inch of slump. In their example, three extra gallons a yard took a 3-inch slump to 6 inches and cost more than 15 percent of the strength. ASTM C94 allows water on site only on arrival, before more than a quarter yard is discharged, and only up to the mix's maximum slump or water-cement ratio. The water has to be measured and recorded, and the drum turned 30 more revolutions. Past that, it's on whoever asked for it.
If the crew needs a wetter mix to place it, order it that way, and ask the plant about a water reducer instead of hosing the load.