In archiving, you are typically setting older content to a read-only state for a period of time so that it can be referred to (but not modified). To do this, follow these steps:

1. As either a global admin, site collection administrator, or SharePoint administrator, navigate to a site (modern) or site collection (classic).
2. Select the gear icon and select Site settings. If the Site settings option is not visible (for example, in a modern site), you may need to select Site Information and then select View all site settings.

3. Under Site Collection Administration, select Site Policies. If Site Policies does not appear, the feature is not activated and you'll need to activate it (select Site Collection Administration | Site collection features, scroll to Site Policy, and click Activate. Close and reopen Site Settings): 

4. Click Create.
5. On the New Site Policy page, enter a name for the policy and select closure and deletion options. 

You can choose to archive a site (closure, in SharePoint terminology) automatically. Configuring automatic site close policies in this interface will also configure automatic site deletion, so it may be advisable to select Delete sites automatically (which requires manual site closure). 

Automatic site closure policies are calculated based on the close event formula Site created date + n <time period>, where <time period> is days, months, or years. Automatic site deletion policies are calculated based on the deletion event formula Site closed data + n <time period>, where <time period> is days, months, or years:

6. Select the checkbox to make the site read-only when it is closed and click OK.
7. After the policy has been created, navigate back to Site Settings and select Site Administration | Site Closure and Deletion.
8. Under Site Policy, select the site policy to apply and click OK:

Site closure policies can be useful in managing the life cycle of your SharePoint environment. Once you have defined a site closure policy and applied it to your site, you can go back to the site settings to close the site manually. Alternatively, if you've configured an automatic closure and deleting policy, the SharePoint Online timer jobs will manage it for you.