Infrastructure is scoped to the website or system it supports, and the work can cover any of the areas below.
Hosting setup. The environment the site or application runs in, sized and configured for that project.
AWS. Amazon Web Services when the project needs cloud infrastructure on AWS, including the pieces required to run and deploy the application.
Deployment. A repeatable way to release a new version, with separate environments where the project needs a place to test before production.
Domains and certificates. The public address and HTTPS, so the launched site is reachable and the connection is encrypted.
Monitoring and backups. Checks that show whether the service is up, and backups so a restore is possible. The exact checks depend on the project.
Security configuration. Access, secrets and the basic hardening of the environment, agreed as part of the setup rather than left for later.
Migration from unsuitable hosting. When the current host is the problem, the site or system can be moved. The plan covers what must keep working during the change.