WannaCry is ransomware that can lock up your data unless you pay the hacker who created it. WannaKiwi, however, finds the crypto key in your PCs RAM to undo the damage. It only seems to work about a third of the time, however. That's why Leo says to make sure you don't get it by altering your behavior, and by making sure you have current backups of your data should it happen. One thing you should never do is pay up, because you don't know if you'll get your data back, or if there's something even worse getting installed.