API stands for Application Programming Interface, and is used to create a connection both between widely different systems, but also internally in one system. APIs are a kind of bridge between softwares so that they can talk to each other and exchange data.
Let’s take an example. When you enter your email to sign up for a newsletter, your email is sent to the website’s database, and then forwarded to a newsletter system via the API. Today, however, there are many CRM systems that have developed plugins for WordPress and receive the email directly.