What is…

I’ve come across something just recently, I’m not sure what it is, and I’m not even sure how to go about “Googling it”. 

As I’ve been working to modify some CSS, I’ve come across an image, with a long number following it. For example:

url(‘../images/background-img.jpg?1487047946’) white;

What is the number for? Attempts to Google it just get responses to “What is .JPG” or the like.

Thank you for the help.

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/2/feed