Bottom paint (anti-fouling paint) is a paint or coating designed to discourage weeds, barnacles, and other aquatic organisms from attaching themselves to (and in the case of wooden boats, eating) the underwater portion of your boat's hull. Bottom paints have traditionally accomplished this by including a biocide, with copper being the most commonly used in today's formulas. In general, the more copper or other biocide a paint contains, the more effective, and expensive- it is.
With hard bottom, or hard modified epoxy paints, rather than the paint itself, it's the copper biocide that gradually wears away allowing the water to penetrate deeper and deeper into the paint. Hard-bottom paints form a tough, hard coating that holds up well and doesn't wear away, making them a good choice for faster boats. They can even be burnished, allowing racers to squeeze out every possible bit of speed from their hull. On the downside, when the copper is depleted, the hard, tough coating remains and it can be messy and difficult to remove, particularly if multiple layers of paint have been added over the years. Eventually this accumulated paint reaches critical mass, becoming so thick that it begins to crack and peel, necessitating a complete stripping of the hull.