It's just like what sholiday mentioned.
Re-confirming that the .chartboost folder can be safely deleted.
From my looking around, it is the cache folder for the ads you see in games with the Chartboost ads monetization platform.
Good Effects in deletion:
- recover some storage space
- get rid of old, unused ads (if any; oldest I found was just June 9)
Bad Effects in deletion:
- playing games with those ads will:
--- slow down ads-loading
--- re-download ads from the internet, which consumes data (around 750KiB to 3MiB per video)
You can check how large the ads are by, well, checking the contents of the folder itself. It should be in somewhere like "/storage/emulated/0/", or basically when you open up your internal storage device for viewing.
You must have the option to "Show hidden files" to be able to see & access the folder.