This step is a strong subject of debate within Wealthy Affiliate of whether it is really necessary. However, it can't hurt your rankings and it's quick so I don't see any reason not to.
The meta description is the description of the page or post in search results on Google. Even if they don't provide much ground as of rankings they certainly can help increase click through rate by getting your readers interested.
You can add a meta description by scrolling below your post and looking for the All-in-one SEO field that says "description". Shoot for a meta description that is 150-160 characters. You can not go above 160 or Google will cut it off. Work your keyword into the description.
Here is my example for the keyword "How to Lose Weight Fast":
Trying to lose weight can be very frustrating with all the scams and lies out there. However, I will show you how to lose weight fast with one simple trick.