What is directory submission? is it useful?

It's an off-page technique in SEO!
And yes, it's useful if done properly.

Directory submission is one of the most commonly used SEO strategies

1. Here are the few solid benefits of directory submission.1. For better search engine rankings
2. Increase link popularity
3. Indexing of pages and blog posts
4. Generate more traffic to your blog
Types of directory submission is as follows:

Free directory submission
Reciprocal regular web directory listing
Paid directory submission
Automatic directory submission
Manual directory submission
Nofollow directory submission
Dofollow directory submission
Niche directory submission