Online vetting means that before somebody joins a group or community on the internet, somebody else checks to make sure they are okay to be there. This is like when you invite friends to your birthday party, you want to make sure they are nice and won't cause trouble so that you can all have fun together.
To do online vetting, the people in charge of the group or community (like the parents for a birthday party) might check the new person's profile or ask them some questions to see if they are a good fit. They might also look online to see if the new person has done anything bad or harmful before.
The reason why online vetting is important is to keep everyone safe and happy in the group or community. Just like how you want to make sure your birthday party is fun and enjoyable with your friends, online communities want to make sure that all their members feel safe and comfortable talking and sharing online.