How do you define a material change in circumstance in custody arrangement

I'm confused. I'm trying to find out exactly what this means: "a material change in circumstance" because I want to see about changing my daughter's custody. I think she should be with me, but I don't know what that material change in circumstance means, and nobody will tell me. I asked the clerk of court, and she said that I had to get an attorney. I don't have money for an attorney. Isn't there just a definition for that? Thank you.