Retraining of object detection models
It will not
It's about using a model which was trained on thousands of Classes on Millions of images of ImageNet.
 Chances are very high that most of the classes you have in your dataset is already there.
In general, if you trained a model on a super-class (e.g. vehicle), then you may reuse it to classify the Car variant(Utilizing its initial layers).
Point is to reuse the characteristics already learned by another model. It can reduce training time drastically.