From Teflpedia

Backpacker is a somewhat disparaging term used by TEFL professionals to refer to untrained university students who do a bit of freelance English teaching to earn an extra bit of beer money.

In the most negative view backpackers are considered to have low costs, little interest in the long-term development of their students and no training or knowledge of methodology. It's claimed they undercut the market with bad, low-cost classes and damage the reputation of the profession.

Another way of looking at it is that they provide a service for students who want a particular type of instruction, typically in a conversation class. It could also be argued that they target a different market sector from that typically exploited by resident teachers.

“Backpacker” Teachers as axes of evil?