what is the difference between < a href = nofollow and = dofollow internal link

Hello, Experts
I create a website using wordpress, and add some on blog post, but i'm
confused about yoast seo plugin funtion link building.
Please share why is internal linking is important and why is it.
 html code < a href works ?

Thanks
 Ryan Khan
Website : https://www.menshairstyleswag.com