So rather than posting an image that is called 143335.jpeg
You call it nigel_slater_cook.jpeg
Google will recognize that name and the link to your image may pop up in searches.
Text assigned to images allows the viewer to understand what they are looking at if the image doesn't display.
In other words it is describing, in words what the picture is saying.
So, alt text for the image above could read: '
portrait of Nigel Slater cook and writer at Chelsea Flower Show 2014 photograph Lynn Keddie'
Not, 'portrait of a man'
How many words?
What do I say?
Be specific and use relevant keywords
How do I put it in?
Alt text is placed in the code.
If you use Wordpress there are plugin's available which make it super easy.
Or, if you use other software such as Weebly there are simple ways to include this information, the programming does the rest.
So that's it, a simple way to boost your search engine rankings and keep Google happy.