Alternative Text is an important piece of making a great looking website accessible. Most people know that including "alt text" is important but many people aren't exactly sure what they should put in this area.
A simple way to create "alt text" is to pretend that you are describing what you are seeing in the image to someone over the phone. You want to be descriptive enough so that they can imagine the image but fairly succinct. Try to limit the description to a quick sentence. Industry standard says 125 characters or less.
Alternative text can help with SEO for a page but it should not be used as an SEO hack. Putting keywords in the alt text to improve SEO without consideration for actually making a coherent description of the image will actually make your website less accessible.