<link rel="stylesheet" href="css/style.css?v=1">
What is it for?
The short answer is to force the update if the stylesheet is in the browser cache. Often v stands for the version.
Of course there’s a long answer.
Often to converse the bandwidth, stylesheets send headers to the browser that they should expire long time later from now, often a year. They also send 304 Not Modified so that the browser can use the cache.
It’s a great feature, but it becomes a problem if there’s a change in the stylesheet and you want to refresh it. The stylesheet is requested from the browser and the asset will send 304 Not Modified. The browser would re-use the cache instead of downloading it fresh. The end user would never download it for up to a year.
You can solve this issue by adding a query string.
<style type="text/css" rel="stylesheet" href="css/style.css?v=2" />
When the file gets updated, you can increase the version. You can do it with template like razor or tokenise it.