The size of an image, Javascript or CSS file in bytes. For example 343k.
In all things web, it’s important to keep file sizes as small as possible. For example, when uploading images to your website, they should first be sized down to the correct dimensions, optimised and compressed to reduce their file size.
The smaller the size of all the files on your website, the quicker it will load.
You’ll often hear developers tell you that file size is too big.