The Meaning
Understandable, have a great day (which you will also come across as Understandable, have a nice day) is a straightforward meme used in situations where one person requests something, gets denied, and then simply replies:
Understandable, have a great day.

While there are many more complex memes out there, the meaning of this one is self-explanatory, and its beauty is in simplicity.
Now, let’s take a look at its origin.
The Origin
It dates back to the beginning of 2017, more precisely – February 2017, and, at first, a seemingly unpopular Twitter post.
What was it all about?
The post was a deep-fried comic (which means intentionally edited to “burn” the image quality) consisting of three panels. The comic features Shaquille O’Neal visiting a drive-through fast-food restaurant and his brief conversation with an employee. The quick exchange that soon turned out to be one of the more popular memes of all time goes like this:
Shaq: Hello, food?
Employee: Food broke.
Shaq: Understandable, have a great day.
Here is how the original comic looked like:

But wait – didn’t we say that the post was somewhat unpopular at first?
That is correct! However, in a matter of weeks, it took over the internet by storm, although in a slightly different form.
How It Became Popular
Even though it originated on Twitter, the meme didn’t gain popularity on that platform.
Soon after @liltusk first posted it on Twitter back on February 26, 2017, a few larger Instagram and Facebook accounts shared the meme on IG, where it started getting increasingly popular.
It soon went through a couple of slight variations, where the version Understandable, have a nice day first appeared.
Around the same time, people started using only the third panel of the original comic (the one with the memorable sentence) as the legendary meme, and it has stayed that way up until today.
How to Use It Properly
There is not much complexity behind the use of the meme, so it would be hard to misuse it.
Next time a friend or an acquaintance responds negatively to your request or a simple question, feel free to respond with the iconic:
Understandable, have a great day.
You might even be able to turn that no into a yes if they know their memes.