Description too long issue means that description tag length on some pages is longer than 160 characters.
Why it is important
Search engines mostly use description to generate a snippet. The snippet quality influences the click-through rate, consequently influencing the website traffic and, indirectly, its ranking.

If the meta description is too long, it can be treated as spam and ignored. In its turn, this will cause loss of opportunities for snippet optimization.
How to check the issue
Using any browser is enough to check the issue. Open the source code of the flawed page. To do this, click the right mouse button at any spot of the page and choose “browse the code” option, or apply an online tool https://codebeautify.org/source-code-viewer.
Find <meta name=description content=””/> and check the meta description length. It is not recommended to use more than 160 symbols for description. To count symbols, you can copy the meta description to any online service for symbol counting, such as Copywritely.
How to fix the issue
If you generate meta descriptions, change the generation algorithm and template to make sure your meta description doesn’t exceed 160 symbols.
If you generate meta descriptions manually, change them for all affected pages.
Detect too long description tags
Audit your website to detect pages that have too long meta description tags