Upside capture is your average return in the benchmark's positive periods divided by the benchmark's average return in those periods. Downside capture is the same for negative periods. A strategy at 85% upside and 55% downside participates in most rallies and dodges nearly half of each decline.
The pair is more informative than either alone, and their relationship maps onto beta asymmetry. Capture of 110 / 110 is just leverage with extra steps. Capture of 70 / 40 is genuine defensiveness. Capture of 95 / 120 is the profile of a strategy that sells insurance and does not know it.
Watch the sample counts. With monthly data over three years there are perhaps 22 up months and 14 down ones, so downside capture is estimated from very few observations and moves a lot with one bad month. See win-rate-confidence-interval for the general shape of that problem.
Related: benchmark, beta, information-ratio, win-rate-confidence-interval