Highroofing essentially consists of getting up onto an extremely high roof or structure where nobody can reach you or easily hit you, and then totally exploit whoring the hell out of that position.
Because with highroofing you can advance to the roof edges unpredictably, quickly pop up, use a weapon, then retreat to total cover and safety, it is considered to be a lame or cheap tactic by many. Prolonged use of highroofing may cause some players to ragequit, which can be hella funny but is somewhat mean.
However, if no exploiting or glitching is involved and it is an intentional game mechanic, it is generally not considered highroofing, though some noobs will accuse people of it and whine a ton about it anyway.
"Damn noob, If you keep highroofing I will ban you!"
"Hes highroofing again omfg"
"Get down from there and stop highroofing you noob"
"Dude how did you get up there?"
"stop the nade spam from up there, highroofing scum"
"learn to highroof lol"
"I cant effing hit the highroofer"