Bottom paint (anti-fouling paint) is a paint or coating designed to discourage weeds, barnacles, and other aquatic organisms from attaching themselves (and in the case of wooden boats, eating) to the underwater portion of your boat's hull. Bottom paints have traditionally accomplished this by including biocides, with the included 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 costly it is.
Ablative, or self-polishing, paint gradually wears away a tiny bit at a time to reveal fresh biocide as your boat moves through the water , in a fashion very similar to how soap wears down with usage. One advantage to this is that as long as paint remains on your hull, you know it's working to prevent growth. Another benefit is that as it's constantly wearing away, there's no buildup of old paint, which can be a pain to remove when the time comes. But ablative paints can be less effective if your boat remains idle for extended periods of time, which denies it that self-cleaning action provided by water movement. The flip-side to this self-cleaning feature is that ablatives aren't a good choice for fast-moving boats, which would accelerate the ablation process and cause rapid paint loss.