Tripwire forms automatically between two tripwire hooks when they are placed along a straight line. Tripwires act as an initiator for various mechanical and redstone effects.
Tripwire hooks are composed of three components, namely a stick, an iron ingot and a block of one of several substances including wooden planks and most types of stone and metal. When the finished hooks are set down in proximity to one another, tripwires automatically come into being between them.
When something passes between the hooks, the tripwire is set off any machinery attached to it via redstone trails is activated. This is their central utility and purpose in the game.