grid in mm or inches says that the grid settings in the schematic/layout/library editor can be chosen in inches or if you prefer in millimetres.
up to 255 drawing layers --- EAGLE distinguishes between signal layers and drawing layers. The professional edition, for example, can have a maximum of 16 signal layers. And there are additional "non-electrical" layers, like some for the silk screen on the board or for solder stop mask and so on. There are some pre-defined drawing layers and there are free layers that can be used for any purpose in your drawing. The maximum number of all these layers is 255.
