Glyphs are enhancements that can be slotted into spells in your Great Mage spellbook to enhance a spell. Each spell has number of Glyph slots on it, from 0 to 3. A Glyph can later be removed from a spell after slotted, but the Glyph is lost and destroyed; it only frees up the slot for the spell to get a different Glyph.
Glyphs can be obtained from monster lairs as random loot or quest rewards.
Note that not all Glyph effects work the same way on all spells; for example, putting a Strikestone of the Elders on a Frost Weapon spell, will not give the target friendly unit the ability to stun, but will instead stun the target of the spell (the friendly unit). Many Glyph effects only affect direct-attack spells; a blessing/buff spell is not affected by Critical Chance, and Damage Additions, such as from Strikestone of Elements, likewise do not have the expected outcome because they affect the spell, not the blessing applied by the spell which is actually a separate entity in the form of a Perk.