AP(Average Precision), mAP(mean Average Precision)
https://medium.com/@jonathan_hui/map-mean-average-precision-for-object-detection-45c121a31173
For object detection, you should set confidence threshold value which will be used for classification. You can make a graph with precision and recall based on difference threshold values. For example, recall will be close to a very low value and precision will be close to 100% with a high threshold value. on the other hand,it will be opposite with a low threshold value.
Average precision means an average value from each precision with different threshold values. By the way, in a graph above, precision values should have a linear type of decreasing continually. So, a maximum precision value should be applied if the value increase suddenly. (Please, check the url above. )
The mean average precision is an average value with all class objects.