The title shows in the tab and on a Google search result.

The description also shows on the results page.

Resources

Title

Length: 20 to 70 characters

From Woorank:

Since they are one of the most important on-page SEO elements you should make your title tags between 20 and 70 characters including spaces (200 - 569 pixels). Make sure each page has a unique title and use your most important keywords. For internal pages start your title tags with your most important keyword(s).

Style guide

15 Title Tag Optimization Guidelines For Usability And SEO

  • [Keyword Phrase] | [Company]

Description

Length: 70 to 160 characters

From Woorank:

Meta descriptions allow you to influence how your web pages are described and displayed in search results. A good description acts as a potential organic advertisement and encourages the viewer to click through to your site.

Keep it short and to the point; the ideal meta description should contain between 70 and 160 characters - spaces included (600 - 940 pixels).

Ensure that each of your web pages have a unique, straightforward meta description that contains most important keywords. These keywords are especially important because they appear in bold when they match the user’s search query (See the Google Preview below).

Check your Google Search Console (Search Appearance > HTML Improvements) for any warning messages to identify meta descriptions that are too long/short or duplicated across more than one page.

Keywords

Example:

<meta name="keywords" content="HTML, CSS, JavaScript">

Note that Google ignores this tag and only uses content.

Source: Google does not use the keywords meta tag in web ranking post on Google Blog site.

Google does also give more weighting to bold and italics words.

Robots

From docs.

robots: the behavior that cooperative crawlers, or “robots”, should use with the page.

It is a comma-separated list of the values below:

  • index
  • noindex
  • follow
  • …

e.g.

<meta name="robots" content="index, follow">