ALT text is simply short for 'alternative text' and is used to accurately describe the contents of an image specifically for when it doesn't load properly.

ALT text is used by those who cannot 'see' the image; whether this is for accessibility reasons (one example would be those who are visually impaired), search engine bots or just for when the image doesn't load properly.

The SEO benefit refers to the fact that search engines will use the ALT text to better understand the content of the image. So ensuring it is accurate, descriptive and contains keywords (only when applicable) is important.

Google is much, much better now at recognising the content of an image without using the ALT text. So first off, make sure the imagery you are using is relevant to the keywords you want to target. Then, back this up, with a nicely written ALT text.

Here's some more information we've grabbed straight from Google - https://developers.google.com/search/docs/advanced/guidelines/google-images