Hoping to avoid the glut of unlicensed products sullying retail shelves like they did during the previous generation of console games, Nintendo has instituted its own seal of quality. It places it on officially licensed games and products that meet the high standards of the company. In other words, if you see something for sale that is compatible with the NES, but doesn’t have this seal, it’s not guaranteed to work and/or hasn’t met Nintendo’s testing standards.
Nintendo’s own words on the subject:
Look for this seal on all software and accessories for your Nintendo Entertainment System. It represents Nintendo’s commitment to bring you only the highest quality products. Items not carrying this seal have not been approved by Nintendo, and are not guaranteed to meet our standards of excellence in workmanship, reliability and most of all, entertainment value.
The NES isn’t even available nationwide yet, so it will probably be quite a long time before any unauthorized products make it to store shelves. That’s assuming the NES is a success. We should know how the test went come Christmas.