What are meta descriptions?
Meta data is an important part of your pages' SEO, with titles that include up to 60 characters of relevant keyword phrases and descriptions that reinforce the page content.
Specifically, meta descriptions are the brief description you'll see below the blue meta titles when performing a Google search. It typically includes one to two sentences that summarizes the content on the corresponding page, and can influence search engine users to visit your page.
The Importance of Rich, Optimized Meta Descriptions
Meta descriptions don't directly affect SEO unlike meta titles and URL structure, they can play a part in encouraging people to visit your website. You should still include a keyword instance if possible to help give Google more context, but you want the description to read naturally and help establish your page as an authority over other results.
Consider the Length
Meta descriptions used to have a maximum length of 160 keywords, but that has been shortened to around 156. However, you should keep in mind that Google doesn't actually measure meta data in characters. Instead, Google cares about the pixel width, which can cause the maximum length to vary. There is a specific reason for this.
Like the content on this page, Google search results use a proportional font, which means that each character takes up a different amount of space; no two letters or numbers are exactly the same width. This is different from monospaced fonts such as Courier, which makes it so letters, numbers, punctuation, and symbols each take up the same amount of space. If your description falls outside of that pixel limit, Google will cut off part of it and replace it with "..."
Don't worry about paying full attention to pixel width, though. Staying within 156 characters will likely guarantee that your full description appears in the results.
Giving each page a unique meta description can help your website appear more legitimate and indicate to both visitors and Google that you're a more reputable source.