Georeferencing is the process of assigning x, y coordinates to a raster file, such as an aerial photograph, scanned historical map, etc. so GIS software can place the resulting georeferenced file in its specified location on a map. The georeferencing process involves assigning control points to the raster image and matching them to control points on an existing georeferenced data set.
Georeferenced fire insurance map of Harvard Square (1885). Map from Insurance Maps of Boston, Vol. 4.
A datum is one component of a coordinate reference system. A coordinate reference system helps GIS software like ArcGIS Pro interpret your data by telling the software where your data is located in the world and how to display your data on a flat surface like a map.
Geographic coordinate systems tell the software where in the world specific features are located via latitude / longitude coordinates. The measurement units are angular (e.g., degrees). Projected coordinate systems tell GIS software how to take the geographic coordinate information and represent it on a flat surface like a map using specific measurement units, directives for how to center the data, etc. The measurement units for projected coordinate systems are usually linear (e.g., meters). Since there's no way to flatten the Earth without some form of distortion, this is where map projections come into play.
Check out the following resources to get a quick intro to map projections and how to determine which map projection(s) are suitable for your study area and analyses.