How do those density test work on the construction site.

Specifically the dirt tests.a person pounds a stake into the ground,then sticks a machine with another stake protruding from it clicks it on an bam water content an density.

It is a nuclear density gauge. The rod on the machine has a nuclear source embedded at the tip. The first rod is just a steel rod, and is used too make the hole for the nuclear probe rod. The denser the soil, the more of the subatomic particles given off by the source are absorbed. The gauge counts the particles and gives a reading of the soil density.

Wikipedia has a good article, if you want more details.

Density test is very important in the construction field, it does relate to safety such as in case of soil compaction for building, roadways, embankments, where a particular density is required for safety. A density test is a type of materials test to determine the density of compacted soil, rock, or other materials, either in the field or in a lab setting. Standards may be set by government agencies or through internal policies, and testing may need to take place before construction can proceed and before facilities are opened to the public, to reduce the risk of safety problems.