FOMO stands for fear of missing out and its the anxious feeling you get when you feel like you are somehow passing up an opportunity. Casually we use FOMO to describe that feeling of being pressured to go to a party we dont really want to go to or to buy that Amazon sale that we didnt really need. However, free-to-play games, and in this case, Mortal Kombat 11 have been using FOMO to get us all to spend more money.
Source: N4G PC How Mortal Kombat 11s microtransactions manufacture FOMO