sstoft

I’m a newbie too, so this is only a clue, not an answer:
put your code in:
/wp-content/themes/yourTheme/functions.php
And where is that? It’s in your WordPress folder — find it with FTP

(But I think it may be safer to make you own my-functions.php,
however I forget how to get your them to read that.)

I thought this was just a fake comment from an enemy. But if you read down the list of comments, you will see that several others do elaborate as esmi requested. So now I think WP/esmi should investigate.
They explain that the plugin inserts its own ads randomly in place of your ads. One claims to have screenshots of this.
If this is true, WP should ban the plugin, but leave it listed in the directory with an explanation and warning to others who might try such tricks. 400,000 downloads is a lot if this is going on.