What is the span tag does?
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>
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)
Thank you everyone!