"HOPOs" is an abbreviation for "Hammer-Ons Pull-Offs", special techniques used in Guitar, about Legato style.

In JamLegend, the system of Hammer-Ons (also written as "HO", more rarely) is featured with notes having a little shiny shape on them.

When you have to play these notes, you don't need to Strum them if you successfully hit the note before it, just hit the key corresponding to the note.

Screenshot of Hammer-Ons on JamLegend, on the song "Open Your Eyes", by Your Favorite Enemies