An alt tag in html, also referred to as an alt attribute, is a text alternative for an image describing what an image depicts.
Despite recent progress, search engines are still pretty basic when it comes to deciphering the subject of an image. The alt tag gives a text description so it's easier for the search engine to decipher.
A useful useful way for an SEO to think about alt tags is in terms of accessibility. Many visually impaired people experience the internet with the help of a screen reader, which scans and reads aloud the text on the page. When it reaches an image the screen reader reads the alt tag in an attempt to explain what the image is about.
Like a screen reader, search engines use the alt tag and image title to understand images so it's crucial to your SEO efforts that your alt tags clearly explain what the image is about. To help the search engines contextualise the page, it's a good idea to use target keywords in the alt tag where necessary – but don't abuse this! Alt tags for images which are purely decorative, in that they don't add any important information that will help the user understand the page, should be left blank. Read more about alt tags and SEO in this blog post.
Alt tags go within the image tags in html as shown in the bold text below:
If you haven't already, you should make sure all non-decorative images on your website have an alt attribute and fit in a target keyword if it's appropriate.