App Photo : process "restauration" running

I

In the native app Photo of my Ipad (Ipad pro, 9,7" wiffi ; IOs 9.3.4), a the end of the album collection, there is sometimes the word "restauration" that appear, with a scroll turning that invites me to wait.

There are both photos synchronized with ITunes and photos transfered with PhotoSync.

Do You know what is this "restauration" process ?

Thanks