hr is a written short form of hour or hours. You see it where space is tight: timetables, recipes, gym plans, science notes — 2 hr, 24 hr. You read it as “hour(s)”; you do not usually say the letters “H-R” when you mean time. It is for writing, not a word you drop into slow careful speech. Though actually, once the letters are capitals, HR is a completely different thing in offices (see watch).