The difference between the Most Popular and Trending recommendation box is that the Trending items are simply the popular items in a shorter period of time. Trending items are the most popular items depending on the most recent traffic on your store (approximately 24 hours) and the Most Popular items are items that are given a popularity ranked from 21 days ago.
Both the Most Popular and Trending algorithms work very similarly. Essentially, the algorithms rank each product based on a variety of parameters including: user clicks, add to carts, purchases, time spent on each product, etc.
To help you choose popular or trending items, take a look at what other brands are doing in our article on product recommendation examples that convert.