Facebook engagement is the combined activity your post has received from such as likes, comments, shares and it can also be extended to by checking for your location or when being tagged.
For example, Harry sees your post and likes it. You have received 1 engagement. A few hours later you decided to add additional content to your post, Harry decides to love your post instead, you have received 2 engagements. Later in the day, Harry decides to make a comment and shares your post with a friend, you have now received 4 engagements. The same person can become engaged with your post and your engagement metric will continue to go up.
The engagement will include all reactions, comments, and shares on the original post and any shared instances. Interactions that have been changed or deleted are counted towards the total, for example, changing a reaction from angry to love counts as two interactions