For what alt it is?

Answer 1

Reff.kevin
The required alt attribute specifies an alternate text for an image, if the image cannot be displayed. The altattribute provides alternative information for an image if a user for some reason cannot view it (because of slow connection, an error in the src attribute, or if the user uses a screen reader).

Answer 2

To display the text in alt="text" if your image has not been upload on the web page :) It is common to write what the image shows, per example "cat" for a cat and so on.