An action level, sometimes also known as a tolerance level, in relationship to the National Organic Program (NOP) means the set limit, at or above, at which the Food and Drug Administration (FDA) will take legal action against a product in order to remove it from the marketplace because they feel it's not fit for consumers. The main goal of action levels is to control levels of contaminants in human food and animal feed.
Action levels set by the FDA are established according to unavoidability of the poisonous or deleterious substances. Action levels don't represent permissible levels of contamination in cases where such contamination is avoidable.
As noted above, when a product hits an actual set action level the FDA may take legal action to remove the product from the market, but if no prior established action level exists, the FDA may still take legal action against the product at the minimal detectable level of the contaminant.
Most commonly, this term concerns organic operations that produce or serve food or use animal feed. To see FDA limits view their Guidance for Industry: Action Levels for Poisonous or Deleterious Substances in Human Food and Animal Feed booklet which lists all action levels established by the FDA. You may also be interested in the Defect Levels Handbook.