A good explainer video takes a complicated product and service and explains it to a 5-year-old in 2 minutes or less.
It has a lot of things really well:
1. It is relatively short (just over a minute)
2. It explains both what the product does and its benefits to users succinctly.
3. The graphics reinforce the explanation but uses subtle easter-eggs and humor to keep viewers engaged.
4. The visuals and color palette are strong correlated to the Dropbox brand.
One downside: It doesn't have a strong call-to-action at the end. However, that may have been a conscious choice by the client.