I work as a mentor for a few of their courses. You're connected to X number of students (you set x yourself). Then as they progress through the course they have the option to send you a message to ask a question or whatever. You also do a weekly check-in with them to see where they're at, if they have any blockers, what their goals are for the next week.
You then get paid (via PayPal) based on how your student interactions went on a per week basis. 0$ for every student who didn't message you, 5$ for 1-9 messages, 15$ for 10+ messages. You get bonuses for them completing major sections as well.
You interact via a mentor dashboard on web or via a mobile app. If you use mobile, you're basically being paid to text some students a few times a week. Pay will obviously vary depending on how many students you have and how good you are at interacting with them. (The better your mentor rating the more students you're allowed to take on). I had 40 students and made 1.4K a month. The work didn't feel stressful or anything. It's pretty easy to land (no formal interviews or anything), you just have to get involved in their slack and PM one of the Udacity staffers. Was pretty easy, plus I enjoy teaching/mentoring.
You can also be a project reviewer rather than a mentor, but not sure how that works.