apex:page

This tag represents a single visualforce page. Any visualforce page should be wrapped inside a single page component tag. 

<apex:page>

//Your visualforce page content.

</apex:page> 

Eample:

<apex:page>

<H1>Welcome to Salesforce tutorial<H1>

<p>This tutorial is useful for to understand Salesforce basics</p>

</apex:page>

apex:page attributes:

Action, apiversion, applyBodyTag, applyHtmlTag, cache, contentType, controller, deferLastCommandUntiReady,  doctype, expires, extensions, id, label, language, lightningStylesheets, manifest, name, pageStyle, readOnly, recordSetName, recordSetVar, renderAs, rendered, setup, showChat, showHeader, showQuickActionVfHeader, sidebar, standardController, standardStylesheets, tabStyle, title, wizard.

Refer this for more details:https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_page.htm