In simple words what is the purpose of <aside> in HTML5 ?

Answer 1

The <aside> tag defines some content aside from the content it is placed in.

The aside content should be related to the surrounding content.

Answer 2

To add to Abhi’s great answer, here’s some more resources to give you an idea about <aside>:
https://www.w3schools.com/tags/tag_aside.asp
http://html5doctor.com/understanding-aside/
https://html.com/tags/aside/