How to optimize a meta description tag?
The content of this tag must be customized on each page of your website. If you can't do it (because of low CMS quality, lack of time...) then it is better to keep it empty.
It is useless to try to overload the content of a meta description with numerous keywords. Currently, Google would not take it directly into account to rank a page on a specific term.
Please note that this does not concern all search engines and that this behavior might change in the future.
The most important when creating this metas is to make the Internet user want to visit your website.
When you are in front of a search engine results page, they have to convince the user in a few words that he will find the most relevant answer on your website. Be “Call To Action” and make them want to know more about you!
If it is hard for you to write this meta description, then you should be careful and keep this tag empty. The search engines will display an extract of the text yet present on your website they consider relevant regarding the user's search.
The number of displayed characters fluctuates from one search engine to another. It is important not to write a description that is too short because it would then be possible to display only one description line. If you don't take two lines, you will occupy less space, suffer from a lower visibility and above all, you will leave more space to your competitors.
In general, writing between 130 and 160 characters is a good compromise. Don't forget the importance of the first 50 characters for mobile. Considering that the maximum size changes, you should regularly do display tests.