An umbrella is the simplest and most affordable light modifier in photography, used to enlarge and soften the output of a flash or continuous source. Like a rain umbrella in shape, it spreads a small, hard light over a broad surface so the effective light becomes much larger relative to the subject, and a larger source produces softer, more flattering shadows. It is one of the first light modifiers most photographers buy.
There are two main types. A shoot-through umbrella is made of translucent white fabric, and you fire the light through it toward the subject, turning the whole umbrella into a glowing source that wraps light around the subject with gentle, spreading falloff. A reflective umbrella is opaque, with a silver or white interior, and you point the light into it so it bounces back onto the subject.
The interior surface changes the character of a reflective umbrella. Silver gives a brighter, more contrasty result with more specular punch and is efficient with light, while white is softer and more neutral but loses a little output. Some umbrellas are deep and parabolic, which focuses the light more and gives more control than a shallow umbrella, and many ship with a removable black backing so one umbrella can work either as shoot-through or reflective.
The same rules of light quality apply as with any modifier. The larger the umbrella and the closer it is to the subject, the softer the light, because the source grows bigger relative to the person. Pulling it back makes the light harder and the shadows more defined. A 60-inch parabolic umbrella close to a portrait subject can rival the softness of a much more expensive softbox.
The trade-off is control. Because an umbrella throws light in a wide, uncontained arc, much of it spills around the room, bouncing off walls and ceilings and raising the overall light level. That spill makes umbrellas fast and forgiving for flattering, even light, but harder to shape precisely than a softbox or other enclosed modifier when you want light to fall on the subject and nowhere else. Umbrellas also catch wind easily outdoors and need sandbagging.
Umbrellas mount on almost any light stand and open in seconds, so they are popular for events, on-location portraits, and anyone starting with off-camera flash. They pair with speedlights, studio strobes, and continuous lights alike, and many photographers keep both a shoot-through and a reflective umbrella in the bag. For tighter, more directional control they reach for a diffuser panel or a softbox instead.
For a first lighting purchase, a single convertible umbrella, a stand, and an off-camera flash cover an enormous range of portrait situations for very little money. Start with the light large and close for soft, flattering results, then experiment with moving it back and around the subject to learn how position, far more than gear, shapes the mood of a portrait.