What does "chance of rain" actually mean?

It's not "it'll rain X% of the day" — it's the odds that measurable rain (enough to register in a rain gauge) falls somewhere in the forecast area during that window. A 30% chance means roughly 3 times out of 10 under similar conditions, somewhere nearby gets wet. Above 70%, plan for rain. Below 20%, you're probably fine leaving the umbrella home.