These are brackets to easily build benches along the edge of a deck. (You need to provide lumber as specified on the packaging.) Almost as easy to build as a normal railing, and it makes better use of limited deck space. More detail is visible on the packaging in the product images, but you'll usually want at least one bracket per ~2' of linear bench space.
More details from the product page:
Easy to assemble
Just add 2x4's or 2x6's for complete benches
Modify a flat surface into seating for the whole family
Space saver design
Limited two year warranty
Normal price is ~$30/pr, recently they've been ~$20/pr on Amazon, now $9.69/pr as an add-on item (and a few even cheaper through AWD).