Think about how many times you have done a query and received the answer without clicking on a link to a web page. Usually, this happens when you type a "what is" or "how to" question.
These "Featured Snippets" are displayed at the top of the search results page, frequently in lew of paid advertising links or just below the ads and above the organic search results. These snippets are directly scraped from websites stored in the Google database of crawled websites. They normally display a link to the webpage providing the content.
A Quote from Google:
"When a user asks a question in Google Search, we might show a search result in a special featured snippet block at the top of the search results page. When we recognize that a query asks a question, we programmatically detect pages that answer the user's question, and display a top result as a featured snippet in the search results."
Are snippets good for my website?
Is this good for you, the website owner? Your content at the top of the search results page, with a link to your website, where you can sell them something or present your message in its entirety? It's easy to answer yes, but, what if the user doesn't click on my link because the Google selected snippet satisfied their curiosity, they will never get to your website. You can influence Google's choice of your website, by using structured data, but you have almost no control over Google's content selection process, other than to disallow it.
I think featured snippets are good.
I think featured snippets are a gift to the website that gets one. Its a way for a lower ranked site to get traffic that may have been lost to the top organic result on the search page. So how do you get a featured snippet selected by Google? Creat quality authoritative content (we have all heard that before) and catalog it with structured data.
40 to 50 word snippets
Write your content in 40 to 50 word snippets, as you should be doing for mobile devices anyway, each preceded by a line of header text of about 60 characters. When writing your text paragraphs have a search term in mind, not any search term, one asking a question, what is ... how to... etc.