A colloquial contraction of ばかり meaning 'only', 'just', or 'nothing but'. Informal spoken usage.
お菓子ばっか食べてると太るよ。
If you eat nothing but snacks, you'll get fat.
彼は遊んでばっかいるから叱られた。
He was scolded because he was always just playing around.
仕事の話ばっかで休まらない。
It's all talk about work; I can't relax.
そんなことばっか言わないで。
Don't just say things like that.