diode
A diode is a one-way valve for electric current. Picture a turnstile that lets people through in one direction but locks solid against anyone pushing the other way — that is exactly what a diode does to current. Push current the "forward" way and it flows almost freely; try to send it backward and the diode slams shut, blocking it. This single trick of letting charge through one way and not the other is the foundation of almost everything else electronics does with diodes — steering messy power into clean one-way current (rectification), shielding delicate parts from a backward surge (protection), and, in an LED, turning that forward flow into light.
The magic lives at a PN junction, where p-type silicon (doped to crave electrons) touches n-type silicon (doped to have spare electrons). Right at the seam the loose electrons and holes pair off and clear out, leaving a thin depletion region — bare of moving charge — that resists current. Forward voltage above about 0.7 V (silicon) collapses that barrier and current pours through; reverse voltage only widens it, so almost nothing passes. That asymmetry is what turns the back-and-forth alternating current from a wall outlet into the steady direct current a phone charger needs — and an LED is just a diode built so that the energy released by its forward flow comes out as light.
The schematic symbol — a triangle pointing into a bar — points the way conventional current is allowed to flow, from the p-side (anode) to the n-side (cathode); the bar marks the cathode, matched on the real part by a printed stripe at the cathode end. A common gotcha: every diode has a reverse breakdown voltage. Push past it and current floods backward. Zener diodes are built to do exactly this at a precise voltage, which makes them handy voltage references rather than failures.