What does `grep -1` do?

I'm trying to make sense of the following shell command, to get a list of packages that can be updated:

dpkg --get-selections | xargs apt-cache policy {} | grep -1 Installed | sed -r 's/(:|Installed: |Candidate: )//' | uniq -u | tac | sed '/--/I,+1 d' | tac | sed '$d' | sed -n 1~2p

What does grep -1 mean? I can find no mention of -1 in grep's man page, or indeed anywhere on the Inteet.