Induction, usually given in seconds, is the time it takes to cast a certain skill. Induction has to be completed before a skill takes effect. Moving in the middle of an induction cancels it. Getting hit in the middle of an induction interrupts it. Many powerful Lore-master, Minstrel, and Rune-keeper skills require inductions, as well as many Creature abilities.
To see inductions on your targets, enable the option "Induction Bar Display" under Options | UI Settings.