You may not want certain pages of your site crawled because they might not be useful to users if found in a search engine's search results. If you do want to prevent search engines from crawling your pages, Google Search Console has a friendly robots.txt generator to help you create this file. Note that if your site uses subdomains and you wish to have certain pages not crawled on a particular subdomain, you'll have to create a separate robots.txt file for that subdomain. For more information on robots.txt, we suggest this Webmaster Help Center guide on using robots.txt files13.
Having a different description meta tag for each page helps both users and Google, especially in searches where users may bring up multiple pages on your domain (for example, searches using the site: operator). If your site has thousands or even millions of pages, hand-crafting description meta tags probably isn't feasible. In this case, you could automatically generate description meta tags based on each page's content.

QUOTE: “Some things don’t change — users’ expectations, in particular. The popups of the early 2000s have reincarnated as modal windows, and are hated just as viscerally today as they were over a decade ago. Automatically playing audio is received just as negatively today. The following ad characteristics remained just as annoying for participants as they were in the early 2000s: Pops up – Slow loading time – Covers what you are trying to see – Moves content around – Occupies most of the page – Automatically plays sound.” Therese Fessenden, Nielsen Norman Group 2017

QUOTE: “Another problem we were having was an issue with quality and this was particularly bad (we think of it as around 2008 2009 to 2011) we were getting lots of complaints about low-quality content and they were right. We were seeing the same low-quality thing but our relevance metrics kept going up and that’s because the low-quality pages can be very relevant. This is basically the definition of a content farm in our in our vision of the world so we thought we were doing great our numbers were saying we were doing great and we were delivering a terrible user experience and turned out we weren’t measuring what we needed to so what we ended up doing was defining an explicit quality metric which got directly at the issue of quality it’s not the same as relevance …. and it enabled us to develop quality related signals separate from relevant signals and really improve them independently so when the metrics missed something what ranking engineers need to do is fix the rating guidelines… or develop new metrics.” Paul Haahr, Google 2016

QUOTE: “If you have comments on your site, and you just let them run wild, you don’t moderate them, they’re filled with spammers or with people who are kind of just abusing each other for no good reason, then that’s something that might kind of pull down the overall quality of your website where users when they go to those pages might say, well, there’s some good content on top here, but this whole bottom part of the page, this is really trash. I don’t want to be involved with the website that actively encourages this kind of behavior or that actively promotes this kind of content. And that’s something where we might see that on a site level, as well.” John Mueller, Google 2016

Another illicit practice is to place "doorway" pages loaded with keywords on the client's site somewhere. The SEO promises this will make the page more relevant for more queries. This is inherently false since individual pages are rarely relevant for a wide range of keywords. More insidious, however, is that these doorway pages often contain hidden links to the SEO's other clients as well. Such doorway pages drain away the link popularity of a site and route it to the SEO and its other clients, which may include sites with unsavory or illegal content.