An ALT-IMG tag is a piece of descriptive text that is embedded within an image tag.
It primarily serves to tell bots what images are, serve as added accessibility for visually impaired users & to appear in place of an image when all of the content is unable to load (slow network connection etc…).
When a bot crawls your site pages it cannot actually see what is in the image, so this text alerts them as to what is in the “blank space” on the page, similar to how a visually impaired user would use it.
How alt-img Tags Work
When a bot encounters an image file (or any other rich media file) it cannot actually “view” what is in the space.
The bots understand that something is there, but that it appears blank.
alt-img tags fix this problem for them as they give details about what is filling that space, similarly to how transcripts are used to tell bots what content appears in videos or audio recordings.
By adding in the alt-img tag, the bot can index the page better for the target keywords, as the image should be relevant to the content.
The image can also be indexed for image search, expanding the potential SERP real estate against your competition.