windows mail folders

[goldcarla]

I have been using windows mail for email for about a year.
something new has appeared in my folders. I now have a + sign on some
of my mail when listed on the screen.
If I click on the + sign it opens other mail titles below.
this has caused me to think I have lost emails.
It happens in all of my folders including sent mail etc.

Help how do I correct this so I do no have these emails combined into
a group listed within another email.