Dogs these days are well treated in many countries. But this wasn’t always the case. Most dogs in the past used to survive on scraps and sleep rough. So a dog’s life was not very good.
This animal idiom means that things are tough, not going at all well.
Her awful husband led her a dog’s life.
Don’t expect a bright future. It’s a dog’s life.
She was living a dog’s life with very little income.