Why it's important
Search engines use your meta data (title tag and description) to display the search snippet for your site in search results. Optimize your meta tags so your site stands out from competitors in search results and entices users to click on your link.
Engaging meta tags will help boost your click through rate and time on site while minimizing bounce rate, which will all help your SEO.
Getting it done
Your title tag goes in the <head> of your page. Use your page's target keyword at the beginning and keep it under 65 characters - any longer and it will get cut off. When added correctly it looks like this: <title>This Is the Page Title</title>.
Meta descriptions are just that - descriptions of what the user will find on the page. Keywords appear in bold so use them here. Descriptions don't affect ranking, but they can affect CTR, time on site and bounce rate. The meta tag looks like this: <meta name="description" content="A short description of your page, around 160 characters."/>
External resources