Water activity, written aw, measures how available the water in a food is rather than how much of it there is. It runs from zero to one, where pure water is one. Microorganisms, enzymes and browning reactions respond to availability, which is why aw predicts stability and moisture content does not.
Why it works this way
Two products can hold the same percentage of water and behave completely differently. Sugar, salt and the food matrix itself bind water and hold it out of circulation. What is left unbound is what a mould spore can use, and that unbound fraction is what the water activity measurement reports.
The practical thresholds are well established in food microbiology. Most bacteria stop growing below roughly 0.90, most yeasts below roughly 0.87, and most moulds below roughly 0.80, with a small group of xerophilic moulds able to grow lower still. Below about 0.60 microbial growth stops altogether and shelf life becomes a chemical and physical question.
What follows in practice
Specify aw, not just moisture content, for any dried, semi-dried or powdered product. The moisture figure is useful for yield and for pricing; the aw figure is what tells you whether the product is stable.
Measure it at a defined temperature, because aw moves with temperature, and measure it after equilibration rather than straight off the line. Moisture, water activity and shelf life sets out the sampling and the packaging interaction.