The play rate is simply the total number of plays divided by the total number of loads. For every load, only one play can be sent - so clicking pause and then play does not count as a second play.
A 50% play rate would mean that half the viewers who visited a page where the video is displayed clicked the play button.
The engagement score is average amount of the video watched per play. A video with a high engagement rate indicates that most people watch the video the whole way through, a video with a low engagement rate indicates that most people only play a small amount of the video and then leave the page or stop the video.
The engagement score includes viewers rewatching the video - so for example if a viewer watches the whole video, then rewinds to half way and watches the second half again, the engagement score for that view would be 150.