Prolactin is a hormone that actually isn't need in men at all. It stimulates milk gland production in females when they give birth. Since men cannot breast feed, they actually don't need the hormone prolactin at all.
Nandrolones (deca, npp, tren) increase prolactin levels in the body. If you use these steroids without an anti-prolactin drug like cabergoline, you can experience high prolactin side effects like gyno, water retention, lactating nipples etc.
Long story short, whenever using a nandrolone, use an anti-prolactin drug like cabergoline. If you need a source, you can order cabergoline from ag-guys.com