system
A chat on subreddit /r/wallstreetbets
human
American options can be exercised at anytime. European options can only be exercised at the time of expiration. This difference causes their prices to vary drastically from each other. 
 Yesterday my vix December call at 20 should have been worth of 600 when the vix was at 26, but it was only worth about $300. The options with October expiration dates were worth about $600.  
 Still made money, but not much.

gpt
Easy way to remember is that you can exercise American options whenever you want because freedom