Georeferenced data are data which have a spatial reference.
It means that, besides their own information, they contain an information about their location. This information of location can be shown in geographic coordinates (longitude / latitude) or in a system of cartographic projection (UTM, Lambert, etc.). Certain software can handle both types of coordinates (as here).
Most of the data used with GIS are georeferenced, but the referencxe sytem used not always clearly documented. Air photos are generally partially georeferenced (usually you know the coordinates of the centre of the image), except when they were the object of an operation of rectification. The georeferencing of the data is sometimes indirect: a list of addresses is not intrinsically localized, but if we also have a ( georeferenced ) street map, then the list of addresses is indirecxtly georeferenced.
The localisation of the data is very important, even determining for the interpretation ("There is a lion" is a fact. " There is a lion in the zoo " and " There is a lion in main street " are the same fact, but because they have different location, their interpretation is very different!).
Various tools of spatial analysis are available to integrate at best the location component in the data analysis. Geographical Information Systems are really the best tools to achieve this.