A Pattern is an item that is used in autocrafting. It stores the recipe that should be autocrafted.
If the Pattern is in processing mode, it will push the inputs to the connected inventory of the Crafter.
The Pattern can be configured to be a oredict pattern. Instead of extracting the exact item, it'll try to extract equivalent items according to the ore dictionary as well.
Use SHIFT + right click in your inventory while holding the Pattern to clear it. You can also override the Pattern in the Pattern Grid.