What does idiom "to have a real axe to grind" mean?
I'm preparing to the exam, and reading about different courses. There are a phrase in a FAQ:

"There are wildly differing reports online about the TEFL course that I plan to take. Who should I believe? How can I make a decision?
Google the usernames of the people who are complaining about the course. Do they seem to have a real axe to grind, for example because they failed the course?"

Please, explain what exactly "to have a real axe to grind" means?

Question

What does idiom "to have a real axe to grind" mean?
I'm preparing to the exam, and reading about different courses. There are a phrase in a FAQ:

"There are wildly differing reports online about the TEFL course that I plan to take. Who should I believe? How can I make a decision?
Google the usernames of the people who are complaining about the course. Do they seem to have a real axe to grind, for example because they failed the course?"

Please, explain what exactly "to have a real axe to grind" means?

Answer

This idiom means to have something to complain about, for ex. Tom, I need to talk to you. I have an axe to grind, also it may mean to have a selfish reason or strong opinion that influences your actions, for ex. The study should be conducted by a firm that has no axe to grind. i hope that was useful.

Answer

When some has a strong persona opinion about somethings; and wants other to accept for selfish reasons.

"Bob is getting the promotion.  I've got an axe to grind with my boss."

To have a strong personal opinion about something that you want people to accept and that is the reason why you do something: