docs: clarify definition of GitHub Actions

The previous definition incorrectly stated that an action is
'a pre-defined, reusable set of jobs or code.' This was misleading
because actions are not sets of jobs—they are reusable units of
code used as steps within jobs.

Updated the definition to accurately describe actions as custom
applications that perform specific tasks, aligning with GitHub's
official terminology.

Author: DanielDerefaka

content/actions/get-started/understand-github-actions.md

@@ -79,7 +79,7 @@ For more information, see [AUTOTITLE](/actions/using-jobs).
 
 ### Actions
 
-An **action** is a pre-defined, reusable set of jobs or code that performs specific tasks within a **workflow**, reducing the amount of repetitive code you write in your workflow files. Actions can perform tasks such as:
+An **action** is a custom application for the GitHub Actions platform that performs a complex but frequently repeated task. Actions can perform tasks such as:
 
 * Pulling your Git repository from {% data variables.product.prodname_dotcom %}
 * Setting up the correct toolchain for your build environment