"Golems are usually described as unthinking constructs and have effectively become synonymous with insentience. Yet only the most powerful mages know how much effort is required to cast a spell on a golem's stone heart, to force the creature to follow even the simplest commands. Renegade mages somehow learned to draw on the complex magic contained in a golem's obsidian heart to produce a powerful mutagenic potion that affects witchers' brains."
When imbibed, Golem's Pith, a mutagenic potion, provides access to a special enhancement of the Intelligence attribute; this upgrade augments the intensity of a witcher's Signs, but only during moonrise.
Note: Despite the item's text stating that consumption "Allows you to buy the Moonrise Intelligence upgrade", the attribute (actually called "Rising Moon") is unlocked immediately and you do not need to spend a talent point.