Difficulty is the number that a dice roll must meet or exceed in order to succeed.
When gathering, difficulty is based on the tier of material being gathered. Normal tier goods will have a difficulty of , meaning that a dice roll must be 5 or higher to successfully gather the item. Each tier increases the difficulty by 5, so the next tier up, Elder, will have a total difficulty of . A roll of 9 or below when gathering Elder will result in a failure.
Every time a player reaches a multiple of the difficulty, an additional item with be gathered. For example, with a difficulty of 5, rolling a 5, 6, 7, 8, or 9 will result in 1 item gathered. Rolling a 10, 11, 12, 13, or 14 will result in 2 items gathered. Rolling a 15, 16, 17, 18, or 19 will result in 3 items gathered, and so on.
The difficulty will be displayed in the drop-down menu of the selected item at a gathering ground location.
When crafting (except components), difficulty depends on the tier of input materials and the selected output tier. If the input and output tiers match, then the difficulty goes up by 5 every tier (just like gathering). For example, crafting Normal with Normal materials is difficulty 5, and crafting Ancient with Ancient materials is difficulty 20.
Your output and input tiers don't have to match. If you want to craft Rare Mana Potions, you can use Normal Hops and Normal Fly Agaric if you want - the difficulty will just be much higher. Conversely, if used Imbued+Imbued to craft Rare, your difficulty would be lowered.
When an ingredient's tier is lower than the output tier, you get a difficulty penalty of +5 per tier. EX: Using Normal+Rare to craft Rare would have a difficulty of 25. This is calculated per ingredient. EX: Using Normal+Normal to craft Rare would have a difficulty of 35.
When an ingredient's tier is higher than the output tier, your difficulty is reduced by 2.5 per tier, rounded down. EX: Using Ancient+Rare to craft Rare is difficulty 13, Imbued+Rare to craft Rare is difficulty 10. Note that the rounding happens per ingredient: crafting Rare with Ancient+Ancient is -2 per ingredient = difficulty 11, not 10.
The difficulty will be displayed on the right side of the crafting menu, but only after all the fields have been filled in.
When crafting a component, the base difficulty of a material depends on both the component and the material. If a Normal material has a base difficulty of x, then Elder is 2x, Rare is 3x, and so on. For example, Jute Cloth has a base difficulty of 6 in Bindings. Using Elder Jute Cloth is difficulty 12, Rare is diff 18, Ancient is diff 24, etc.
The total difficulty is the sum of the individual difficulties. EX: You want to make a Fabric using Imbued Jute Cloth (base diff 4) and Rare Nightweave (base diff 5). The total difficult is 5*4 (Imbued Jute Cloth) + 3*5 (Rare Nightweave) = 35.
1) Mastery. For gathering and crafting , mastery levels 5/13/19 give you -1 difficulty each. For gathering, this also means you need to roll much less to achieve multiples. For example, a player with level 5 Copper Ore Mining mastery would have the difficulty of mining normal copper ore reduced from 5 down to 4. A roll of 4 would then give one ore, a roll of 8 two, a roll of 12 three, and so on.
2) Housing. Each skill has a Housing addition that will reduce difficulty. Like Mastery, this significantly reduces the difficulty for gathering multiple items in one tick.
3) Tools. One of the tools that gathering professions can use has a chance to reduce difficulty for a roll. Note that unlike Mastery and Housing, this difficulty reduction does not get multiplied for higher tiers. For example, if a player is gathering Normal Copper Ore (difficulty 5), and their tool procs a -1 difficulty reduction, the difficulty threshold would be reduced to 4 for the first item, 9 for two items, 14 for 3, and so on.
Difficulty can never be reduced below 2 (although the game may display values <2 for certain combinations of input/output tiers when crafting).