That’s the number of American soldiers who have died in Afghanistan since the inception of the war. The New York Times has an excellent infographic that breaks down the numbers further by branch, province, age, and so forth.
Most telling, in my view, is the graph which demonstrates that the most violent years come after 2008, when we of course saw a policy shift from the Obama administration away from Iraq and focused on “winning” the war in the Afghanistan. As you can see from the graph, the 2009 troop surge was consonant with the heaviest period of American casualties in the history of our involvement in Afghanistan:
More at the link.