Take 20 refers to the ability to assume the best possible roll when attempting to use a skill. (The name comes from higher rolls being better than lower, and from the use of a d20 for skill checks, hence a "20" being the best possible roll.) This simulates a character's ability to take the time required to perform the skill well. In principle, this should encompass multiple attempts, some of which fail, but NWN treats taking 20 as a single attempt, and the time and resources consumed reflect this. (For example, only one healer's kit is consumed when taking 20 on a heal skill check.) Essentially , taking 20 in NWN means replacing a die roll with a fixed value of "20".
Taking 20 is handled automatically by NWN. When a character and nearby party members are not in combat — and, if applicable, when the target is not in combat — that character will "take 20" whenever using the animal empathy, disable trap, heal, open lock, and set trap skills. When in combat, a character with the skill mastery feat will "take 20" whenever using the disable trap, open lock, and set trap skills. (These are the only times a character is allowed to take 20.)