Why some users are ''Pending Approvals''

Hi,

I have some users who are in the section Pending Approvals and I don't know why. They don't have any usernames and there is none in my Joomla users manager. Their user names are : ''Invité'' sorry my site is in french so it would be translated as "guest". I don't want to remove them but also they are stuck there...

Can you please tell me what to change or to do to avoid this situation ?

Thank you