One of potentially many ways of persisting the same set of data. At 10,000 feet, a project can be represented as either a Blu-Ray, or terabytes of models, animation, audio, renders and settings.
Representations are relevant mainly when using the same set of data in different software.
For example, that high resolution hero model works great for renders, but less so for an asset browser. There, a smaller more accessible file format is preferable, such as a PNG. Perhaps a proxy version, such that it can still be rotated, but stored as an OBJ rather than ABC for simplified parsing in the Python based application.
The idea is that, whenever you save data, you save it in every way you can think to use it. Every model is not only saved as an MB, even though it will primarily be used in Maya. It is also saves as an OBJ, because you never know if you someday will want to open it in Mudbox. It's saved as a PNG, because you might not have access to either Maya nor Mudbox when time comes to simply look at the model.
Think of it like portrait. The subject - such as a person - remains the same, but for every portrait strokes and colours differ slightly. Some are caricatures, some are line drawings. Some might be sculptures, or even 3d scans. Vastly different results, each representing the same subject. The same data.
In this case, the subject could be a model, whereas a representation could be an exact representation stored as an .obj, or an approximate one, such as a .png or poly-reduced alembic. Same data, different representations.