If we forgot to write meta tags, especially meta title and meta description, how will search engines crawl and index?
There are too many meta tags available for different usages, but Google understand or you can say use only few.
When Googlebot can't find meta title and meta description in your page then they issue warning to your webmaster/search console tool. But to make proper index of your pages in search results, they will use heading(like h1,h2,h3) as title for their snippet, and any text paragraph for description.
Google will index web pages that don't have any page titles or meta tags. However, your site will usually perform better in search engines if you add them.
The title tag should describe the page in 3 to 12 words. It should use the main keywords for which you want the page to rank. It should end with your brand name. eg
Blue Widgets for Sale - Bob's Emporium
Google indexes the words in the title and assigns a lot of ranking weight to those words. The title shows up in the search results, so it is also very visible to users.
If the page title isn't present, Google will use something else from the page in the search results. Possibly a heading and your brand name from your home page.
The meta description should be a full sentence and should include a call to action. eg.
Low prices on blue widgets, sustainable sourced from around the globe, buy yours today.
Google doesn't index the words in the meta description, but it does show as much of it as fits in the search results when it contains the search phrase.
Google only shows the meta description to some users, so it isn't nearly as important for SEO as the page title. However, when Google does show it, it can be a powerful tool to describe your page in a way that interests users and convinces them to click from the search results to your site.
If you forgot to include page titles and meta descriptions when you launched your site, they can be retrofitted at any time. You will get the SEO benefit of having them soon after adding them. There is no penalty for not having them earlier.
Google's John Mueller says:
Good titles & descriptions are some of the easiest wins you can get on webpages. We did a round of site-reviews for some NPOs recently, and the number one item that came up for almost all of them was that what they cared about wasn't front & center on their pages, and reflected in their meta-data. How are search engines supposed to guess what you want your pages to rank for?