Datums (sets of data) are the basis for all geodetic survey work. They act as reference points in the same way that starting points do when you give someone directions. For instance, when you want to tell someone how to get to your house, you give them a starting point that they know, like a road or a building. Geodesists and surveyors use datums as starting or reference points when they create maps, mark off property boundaries, and plan, design and build roads, bridges, and other structures.
Another way to think about a datum is as a set of information that acts as a foundation for other data. For example, when a skyscraper is about to be built, the construction team must first pour the foundation. Without this element, the skyscraper would be unstable and unsafe. This is the same concept as a datum. While a datum is a mathematical and geometric concept, it acts like the concrete foundation of a skyscraper. Once the foundation is set, the construction workers can build on top of it, creating the building's structure. After the building is complete, offices or apartments can be created inside the building. If the structure is an apartment building, its tenants can bring in furniture and decorate as they please. Although the foundation of the building probably isn't the first thing on the minds of the tenants, without it, the building would not be a safe place to live.
In geodesy, two main datums create the foundation for navigation and transportation in the United States. These datums -- called the horizontal and vertical datums -- make up the National Spatial Reference System (NSRS). Geodesists, surveyors, and people interested in precise positioning use the NSRS as their foundation for reference.