There are four kinds of sickles. Better sickles reduce the chance of failing when gathering a node. This also depends on the level of the node, so a Pristine Sickle will practically never fail on low-level nodes while still failing sometimes in late-game areas.
Herbs are gathered from special nodes, which appear all over the map. The nodes are indicated by floating green orbs. Any node may be represented by one or multiple orbs, scattered around an area. To harvest, approach any orb and press the Gather key. The collected herb will go to the Crafting tab of your inventory.
Some nodes provide quest items, while others contain plants that can be used in Alchemy.
Every node will contain a random herb, chosen from a Tier of herbs allowed for that area. You can use a Kit of Flora Distorters to change the type and number of herbs that you have received.
Higher skill level allows gathering of more advanced nodes, so if you can't interact with one you'll have to find lower-level nodes and increase your Herbalism skill by gathering them.
Your skill level is capped at your level (for example, at level 9 you can have skill up to level 9)
The herb you'll get will have one out of four different qualities: Damaged, Wilted, Mature and Potent. Each quality is potentially more useful than the previous one (but not always, depending on potion recipe).
Harvesting a herb node will deplete it, and it will not respawn for a very long time (except quest nodes). The respawn time is 30 minutes, plus you need to leave the area at least once (or relog) or it will not respawn.
There is a chance of failing when you harvest a node. In that occasion, you will not receive any herbs, but one orb will disappear. If a node has only one orb, it will be depleted, while a node with multiple orbs will allow you to try again. Better tools can significantly reduce this effect.
Herbs can be grouped into certain categories (or tiers), since they are gathered from the same areas, require the same skill level, and have the same base value. Low-tier herbs can be gathered in the starting areas of each faction, while more advanced nodes can be found in later areas:
Tier 1 nodes produce 2-6 herbs for every gathering attempt
Tier 2 nodes produce 2-7 herbs
Tier 3 nodes produce 3-9 herbs
Tier 4 nodes produce 4-10 herbs (5-11 herbs in Yazes Shard and Gipat)
Each area requires a specific range of skill level to allow gathering. In Tenebra for example, some nodes require a minimum of 190, while others need skill level up to 240 to be gathered. Each area can increase your skill level high enough to allow gathering in higher-level areas.