Red wine extract concentrates polyphenols from grape skins and seeds, especially resveratrol and flavonoids. These compounds act as antioxidants. Most of the science looks at these active ingredients, not at drinking red wine.
Capsule or tablet forms supply the polyphenols without the alcohol that would come from a glass of wine. People take the extract with a meal to help the body absorb it.
Dose strengths differ across products because extract concentrations vary. The extract can thin the blood mildly. Anyone using blood thinners or antiplatelet drugs should talk to a doctor first. Pregnant and nursing women should skip concentrated extracts since safety information is thin.