What is use of alt-""/ attribute

Answer 1

Its the alternative text just incase the image doesnt show up on some browsers. You put the text inside the quotes. Also make sure its alt=" " because a dash wont work. hope i was helpful to you :)

Answer 2

as aquarius told but i'll add that it help in SEO too.

Answer 3

Aquarius told correctly alt attribute is an alternative text for a image in case image is not available the /, dash is <IMG...../> is not with alt it is because it has no closing tag but some browser search it so / closes tag.