Alpha is a computer graphics term for opacity. For any pixel on a screen, there may be 3 color channels (RGB) and one optional alpha channel which tells how transparent the pixel is. A value of 0 means the pixel is completely clear. Anything underneath that pixel will show through. A value of 255 (the maximum) means the pixel is completely opaque. Nothing underneath will show through. Intermediate values will result in the pixel's color mixing with whatever is underneath it.

This image shows a purple bar across a painting with varying alpha values from 100% (fully opaque) to 0% (fully transparent).

Sample image: "The Birth of Venus," painted by Sandro Boticelli, c. 1485–86.