An umbrella is a type of modifier that diffuses soft lighting. It’s often considered one of the most affordable and versatile types of modifier. The main difference between umbrellas and softboxes is that while softboxes produce directional lighting, umbrellas create what’s called “inefficient lighting,” meaning it’s spread in many different directions.
Using a flash without a modifier such as an umbrella will likely result in hard lighting, which means you’ll get dark shadows. With soft light, shadows are very light or non-existent.
A common way to use an umbrella is as a “shoot thru.” The umbrella is placed in front of the flash, and the flash shoots the light through the umbrella and onto the subject. This will create even lighting across the subject. You can also use it as a reflector by pointing the flash at the inside of the umbrella and aiming both away from your subject.
You can experiment with shadows and different lighting effects by moving your umbrella around and placing it at different angles. The “classic” position is 45 degrees up and over to one side. If you’re new to light modifiers, you may want to start here and then try experimenting with new positions. Generally, the larger the umbrella is and the closer it’s placed to the subject, the softer the lighting will be.