I have tried googling but didn't find anything. Thank you very much for any help!
I believe this means to play in 7th position: so that the first (index) finger plays on the 7th fret during the part below the horizontal line. In particular the last two notes should be played with barré on the 7th fret.
This is a fingering suggestion, you may follow it or not, depending on what feels better to you.