What helped me out the most when it came to fully understanding promises was simply repetition and going back to look at previous assignments. When we had to write multiple promises for each of our assignments, that helped solidify the concept in my mind. I also did a bit of outside research on the web to help myself understand some things I was foggy on.
Here are a few sites that helped me with my understanding:
Promise (from the MDN site)
Overall, after completing a few different in-class projects, homework assignments, and a group project, I have a better, more practical understanding of promises.