positive and negative will be attracted to each other.
When using a galvanic machine you apply a charged gel to the skin (specific to skin type).
Usually your rollers are negative (client holds the positive to make a circuit) and so most galvanic gels are formulated to use on the negative.
So all you have to do is choose the right gel for the skin type you are working on.
For example you want to add moisture to a dry skin:
Your rollers are negative so your hydrating gel is negatively charged, therefore moisture from gel is pushed into the skin.
very simply put but hopefully enough to jog your training memory.
If the gel/cream is negative, and the rollers are negative, then the gel will get "pushed" into the epidermis of the skin, as it will be repelled away from the rollers.
If the rollers and gel were not both negative (or both positive), the rollers would attract the gel away from the skin, instead of into it.
Also bear in mind that using the galvanic rollers on negative will create a slightly alkaline reaction on the skin's surface, causing the skin cells to swell and be slightly more absorbent for any products. (naturally our skin is slightly acidic - remember the acid mantle?). So if you use negatively charged rollers and product this will help the product to penetrate into the epidermis. More effective than just rubbing it into the skin.