A homeroom teacher is the primary leader of a particular group of students. When the students are not in their regular classes, the homeroom teacher will lead them in other activities such as morning announcements, club activities, free time, etc. For example, I come from America, so the homeroom teacher would usually lead my class for the first 20 minutes of the day in order to give announcements or instructions about activities coming up in the day. Students usually see their homeroom teacher at the beginning and at the very end of the school day. Depending on the country, homeroom activities may differ, but in general terms, this is how I define "homeroom teacher." I hope this helps. Please let me know if you need further clarification.