Bib overalls are an item of clothing often made of denim material. They are like a pair of pants with a front panel (bib) and the back forms two suspenders to cross over the shoulders to attach to the front panel/bib. Stylish women wear their bibs without a shirt or bra, thus allowing voyeuristic sideblouse views.
A skirt may have a bib and suspenders attached as well. those are simply called, "Bibs," rather than overalls.