Thinning ratio is the proportion of paint to thinner in your cup. Most bottled acrylics need thinning to spray — the folk target is “consistency of skim milk,” often landing around 1:1 to 2:1 paint-to-thinner, but the honest answer is per-paint and per-pressure.
Too thick spits and orange-peels; too thin runs and spiders. Thin incrementally, test on a spoon or scrap, and write down what worked — your future self will spray the same paint again.
See it in context in our paints & primers guides, or explore the tools hands-on in The Bench.