It's a diploma that's supposedly equivalent to a high school diploma. By law, this is so. However, if you think about the de facto, this might not be so.
If you're failing high school (or dropped out already), I suggest you drop out and go for this diploma. If not, just go graduate. It's better. Also, if you arn't doing well/didn't do well in HS, you might wanna pursue this degree as well.
GED 1. general educational development. 2. general equivalency diploma.