A triac is a semiconductor device used for AC power switching and phase control applications. Essentially, it acts like 2 SCRs (Silicon Controlled Rectifiers) connected in inverse parallel.
Used as a solid state replacement for a relay in lower power applications. Probably the most common place an electrician would find one is the incandescent lamp dimmer, where a triac is used as a phase controlled switch, to vary the effective voltage applied to the lamp.
