A wordmark is a type of logo. It identifies the company or product with a text-only treatment of its name, often with distinctive characteristics.
Here's an analogy.
If a person is a brand, and their logo is their face, a wordmark is a type of face. The facial features may be rounder and softer, or they might be narrower and sharper.
Why does it matter?
Your logo is one of many ways a user or customer might remember your company and brand.
How does it apply?
Wordmarks emphasize the name of your company, helping commit it to memory for prospects and eventual customers.