- Not available on that edition (the reason is in the tooltip)
- Depends on the setting, your class or another addon (details in the tooltip)
What it does
Checks if the glyph you choose is active in one of your glyph sockets.
Why you might use it
Check whether a particular glyph is active, so a rotation follows the glyphs you actually chose. Many Mists of Pandaria glyphs change how a spell behaves, so a step that only makes sense with a glyph can require it, and a switch condition can pick a rotation built around it. Only available on Classic progression (Mists): Era and TBC have no glyphs, and Retail and Forever use a glyph layout the addon does not read. See also Talent.
Example
you have Glyph of Mind Spike active
Settings
What each setting in the condition editor means, from the game's own help.
Item - The glyph, entered as its item ID or item name; the icon beside the field shows the glyph selected. Only glyphs of your class are accepted. The condition matches while that glyph is active in one of your glyph sockets.