Channeling is a magic skill available to all casters, priests, and hybrids (except bards). It helps to ensure that spells can still be successfully cast even while being engaged in melee by an opponent.
Mechanics[]
Channeling has a chance to gain a skillup on every successful cast of a spell, and is therefore easy to keep current with one's other casting skills while leveling.
Having a high proficiency in channeling decreases the odds that melee push and other movement will interrupt one's spellcasting. The exact formula for this is not known, but it can be observed that smaller amounts of received push appear to make the channeling check more likely to succeed, while being shoved around harshly (e.g. because of taking melee hits from several mobs at once in an open area) makes channeling less likely to succeed.
When channeling successfully prevents interruption, the message "You regain your concentration and continue your casting" will be shown in one's chat window as the spell resolves.
Note that channeling will not prevent interruption as a result of getting stunned. For help with that, consider the Persistent Casting AA and/or stun resistance effects.
Strategic Impact[]
The channeling skill works very well for the classes that have it, allowing them to go toe-to-toe with one or two mobs while retaining a reasonable rate of spellcasting success (apart from cases of actually becoming stunned).
The lack of channeling skill on pure fighter classes can make it very difficult for warriors, monks, rogues, and berserkers to activate clickies with a casting time while tanking a foe, since without channeling, even the tiniest amounts of melee push will interrupt spellcasting every time. At level 66+, the Concentration archetype AA can be purchased by these classes to give them a channeling-like resistance to such interruption.
Prior to that point, it is usually best to activate clickies with a cast time (e.g. the Rune of Healing or Vicious Rabbit prior to starting a fight (or in the case fo the rabbit, as one's first action in starting the fight), since activating them may become difficult or practically impossible once the fight is underway. If someone else will be tanking, rooting, or otherwise controlling the mob(s), this ease of interruption becomes easier to work around. Berserkers also have the option of snaring their foe(s) and kiting away a short distance to activate the desired clicky.
Bards do not have the channeling skill, but have a hard-coded mechanical exception built into their class so that they do not need it: bards will never have their casting or clickies interrupted by pushback or by moving around while casting, making their casting quite unique among all of the game's classes.