What Unity stores there
Library is generated project data. As a project changes, Unity stores imported asset data, script-compilation artifacts, shader-related data, and platform-specific cached information there.
Switching build targets, importing assets, changing settings, and iterating on a project can all add or regenerate cached data. That is why the folder can become large over time, especially across multiple projects.
Can it be removed?
For a normal Unity project, Library is not where source assets and project settings live. When the project is closed, deleting Library makes Unity rebuild the needed generated data when the project is opened again.
Keep Assets, Packages, and ProjectSettings safe. Reopening after removing Library can take longer because Unity must reimport and regenerate its cache.
Why this matters in practice
Understanding Library makes backups, source control, and client delivery more manageable. It helps separate the files needed to reproduce a project from generated local data that Unity can recreate.