Definition
The alpha channel is a special lane of data in a digital image that records how transparent each pixel (the smallest dot that makes up a screen) is. It adds a fourth value, transparency, to the three pieces of color information, red, green, and blue (RGB), so a picture can be laid naturally over any other background.
The magic of laying clear cellophane over a picture
Think of how a cartoon gets made: the character is drawn on a clear plastic sheet (a cel) and placed over a background painting. The empty plastic around the character is completely clear, so the background shows right through. On a digital screen, the alpha channel plays the role of that clear sheet.
A picture on a computer screen is made of countless tiny dots, called pixels. Usually, a color is made by mixing three kinds of light: red (R), green (G), and blue (B). This is called the RGB color model. But those three values alone can't express "see-through" itself.
So computer graphics developers added a fourth slot for transparency after the color information. That's alpha (A). No matter how vivid the color is, if the alpha value is 0, it shows up on screen as completely clear, empty space.
From 0 to 255: how transparency is controlled
The alpha channel isn't just an on-off switch of "open" or "blocked." Computers usually express transparency as one of 256 fine steps, from 0 to 255.
At 0, the pixel is fully transparent and the background behind it shows through 100%. At the highest value, 255 (or 100%), it's fully opaque and completely hides whatever is behind it.
A number in between, like 128, creates a natural semi-transparent effect. Graphics that show through partially, such as a glass window, a water droplet, or a ghost character in a game, all rely on a technique called alpha blending, which mathematically mixes the front color and the background color in proportion to the alpha value.
To be a bit more precise: why does it differ by file format?
You may have downloaded images from the internet where some had a solid white background and others had a clean, see-through one. That's because file formats differ in whether they support an alpha channel.
JPEG (JPG), the format most often used for photos, has no room for an alpha channel at all. So it can't store transparent areas, and when you save, they're usually filled in with a solid color such as white or black.
Formats such as PNG and WebP, on the other hand, preserve the alpha channel fully. That's why graphic designers use PNG when they make logos or character stickers. The gray-and-white checkerboard you often see in image editors is a visual convention, shown only on screen as a temporary stand-in so that people can tell the image is transparent.
Common misconceptions
The gray-and-white checkerboard you see behind a PNG file is the alpha channel itself.
The checkerboard is just a guide the program paints temporarily so users can recognize "this is transparent empty space." The actual alpha channel data is stored only as transparency values from 0 to 255 that you never see on screen.
Where you meet it
The semi-transparent black box that sits faintly behind subtitles in a YouTube video
The invincibility effect in a game, where a character flickers and turns semi-transparent after being hit by an enemy
A PNG website logo with a transparent background, so it sits naturally on top of any background color
In one sentence
The alpha channel is data that adds transparency (A) to the three RGB primary colors, so an image can be blended naturally with any other background.