HTML Unclosed Tags and Meta Tags

HTML tags are like keywords which defines that how web browser will format and display the content. With the help of tags, a web browser can distinguish between an HTML content and a simple content. HTML tags contain three main parts: opening tag, content and closing tag. But some HTML tags are unclosed tags.
Unclosed Tags
Some HTML tags are not closed, for example br and hr.
<br> Tag: br stands for break line, it breaks the line of the code.
<hr> Tag: hr stands for Horizontal Rule. This tag is used to put a line across the webpage.
Meta Tags
DOCTYPE - tells the browser which version is used by this page.
Title - Used in head tag. This tag is used to define the title of html page. This tag is useful for SEO purpose. 
Link - This tag is also used in head section of html page. This tag is used to link files like css etc.
Meta - This tag is important for seo purpose. It tell the search engine about some properties of the current page like. Keywords, Description, Robots, Theme color etc.
<meta name="keyword" content="Keywords of page goes here"></>
<meta name="description" content="Description of page goes here"></>
<meta name="robots" content="index, follow"></>
Style - This tag is used to write css for styling the html page.
<style>
.hero{
background-color:#fff;
}
</style>