What is the span tag does?

Answer 1

You can use it to stylish some selected elements.

Copy & paste this to a web code to see how it looks.
<p>Hello <span style=color:red;>There</span></p>

Answer 2

Basically it's the same like a <div> but an inline element instead of a block element.

It's often used to style certain words within a block differently. I.e. make some words within a <p> red. (See answer of Alex)

Answer 3

Thank you everyone!