Additives serve a similar function to mediums. The difference is that additives don't contain acrylic binders (like mediums do) so you must follow the label and use them in correct proportions. Most mediums can be added to the paint in any amount until you've the achieved the desired effect, but it's possible for additives to weaken the color and adhesion of the paint when overused. Golden offers these two additives which are an effective addition to any painters experience.
Acrylic Flow Release - an additive used to reduce surface tension of the water in the acrylic emulsion, thereby increasing the slickness and flow of the paint. It is effective for achieving rich stains on a porous surface. Not for use by children.
Retarder - an additive used to increase the open (drying) time of acrylic paints. Useful for wet in wet techniques and reducing skinning on the palette.