You may have noticed that in search result pages (SERPs) of Google, there is a two line description:

Google uses (most of the time) your meta description (if it thinks it’s more relevant) and if your description is too long, it will cut it and show 3 dots at the end:

If you don’t want this ti happen, write a good description that is only 155 characters long, for example, in this case the description fits perfectly:

So when writing a meta description remember: (in no particular order)

1 – Write a meta description that is 155 characters long. (or less, obviously)
2 – Write a meta description that is descriptive.
3 – Write a meta description that is provocative.
4 – Use your primary keyword(s) in it, don’t write a stream of keywords, write something meaningful.