Micro-organisms require water to thrive. Many regulatory bodies have established limits to ensure material is properly dried and cured to limit microbial growth.
While related, there are inherent differences between moisture content and water activity. By definition, water activity is the partial vapor pressure of water in a substance divided by the standard state partial vapor pressure of water. In other words, water activity is a measure of water in a substance that is available to react with or attach itself to another material (i.e. available for microbial growth). Many states set the water activity limit at 0.65 because anything greater than this level promotes microbial growth.
Moisture content, by definition, is the amount of moisture in a product and usually reported in percent (%). Oftentimes, it is calculated by heating the material in question and recording the beginning and ending weights to determine loss on drying.