What is the .idea folder?

There is no problem in deleting this. It's not only the WebStorm IDE creating this file, but also PhpStorm and all other of JetBrains' IDEs.

It is safe to delete it but if your project is from GitLab or GitHub then you will see a warning.

As of year 2020, JetBrains suggests to commit the .idea folder.
The JetBrains IDEs (webstorm, intellij, android studio, pycharm, clion, etc.) automatically add that folder to your git repository (if there's one).
Inside the folder .idea, has been already created a .gitignore, updated by the IDE itself to avoid to commit user related settings that may contains privacy/password data.

It is safe (and usually useful) to commit the .idea folder.

It contains your local IntelliJ IDE configs. I recommend adding this folder to your .gitignore file:

# intellij configs
.idea/

When you use the IntelliJ IDE, all the project-specific settings for the project are stored under the .idea folder.

Project settings are stored with each specific project as a set of xml files under the .idea folder. If you specify the default project settings, these settings will be automatically used for each newly created project.

Check this documentation for the IDE settings and here is their recommendation on Source Control and an example .gitignore file.

Note: If you are using git or some version control system, you might want to set this folder "ignore". Example - for git, add this directory to .gitignore. This way, the application is not IDE-specific.