You may have come across some labels in Japan which look like price reductions, but you can't read them. Here's a guide.
The two types most commonly seen are this one:
And this one:
The first, which reads 半額 or hangaku means "half price". That's that solved.
As for the second, the number represents how many units of 10% discount have been applied. In the picture it reads "1割引" (1 waribiki), which equals 10% off. "2割引" would be 20% and so on.
Well, that was easy. Hopefully that will improve your shopping experience.
The most important part is when to go shopping in order to see a lot of these. From my experience living in a few places around Japan, the shopping market will put these stickers on their お惣菜 (osouzai, that's preprepared deli foods like karaage, ebi-fry, tonkatsu, etc.) after the evening shopping rush, or from about 6:30 to 7:30 p.m.
Most of the locals will also know what time to expect the discount stickers to show up, and you'll see them pushing their carts slowly through the deli section waiting for the sticker guy to show up. After he comes through and marks everything for discount, 80% of the stuff that just went on sale will disappear within about 10 to 20 minutes.
You need to act fast in order to get something good. But, お惣菜 isn't the only part of the supermarket where discount stickers will be placed daily. Check out the sushi section, and the fish and meat departments. Ground beef or milk that is marked to expire tomorrow, for example, will also often have a discount sticker