Glyphs are a new type of loot that can upgrade gear to add more stats and special properties. The gear in question has to be level 51 or higher and match the type of stats that the glyph is offering. Each piece of gear can be expanded to include three types of glyph: offensive, defensive, and special. Glyphs themselves can be improved with a component that drops only in the new Dominion battleground.
There looks to be several different ways to get your grubby mitts on glyphs. They can drop, you can do a quest to buy a glyph box, you can buy them from an NPC if your reputation is high enough, and some crafters are able to make them.