Processors handle binary numbers while humans use denary. Binary to denary conversion aids understanding of data representation. ASCII represents text. Bitmapped graphics are stored as binary. Vector graphics store objects as a list of attributes.
A bit mapped graphic is a 2D array of pixels. The total number of pixels in an image is known as the resolution, and the number of colours that can be used is determined by the bit depth.
Here is an example of a black and white graphic. A white pixel is stored as a 0 and a black pixel is stored as a 1.
There are 100 pixels in this image, and each pixel is stored using a single bit. However, if the image included colour, it would be necessary to store more than one bit per pixel. The more bits you allocate to each pixel, the larger the file size.
Bit depth is the term used to describe the number of bits used to store each pixel in an image.