Actually it's called "one-on-one" and it refers to carring a tutoring out between two people: a professor/teacher and his/her student. By carring an one-on-one session out they're able to talk about all the questions, doubts, wonders, and anything that the student wants to know or do. It's so good because the respective teacher will pay totally attention to that student, and will help the student to reach his/her aims about language learning. That's why it's good to pay jsut a little bit of money and make a premium account in order to have one-on-one tutorings with informal teachers or professional teachers (with a so cheap price in most of cases). I really recommend it :D
I hope I'd helped you!