YOLO is a very good and approachable object detection technique. I recently re-read the paper for the original YOLO [1] from 2015 and loved the apparent simplicity of this technique.

As a shameless plug, I wrote an intuitive guide to understanding SSD (Single Shot Detector), another popular object detection technique: https://towardsdatascience.com/understanding-ssd-multibox-re...

[1] https://arxiv.org/abs/1506.02640