Views are the amount of times a video has been watched by other users. They are essential to how YouTube works.
Views have been around since the beginning of YouTube. The more views a video has, the more money a YouTuber is likely to get, because advertisers pay to have their ads shown during videos of those with high view counts.
Originally, the term "viral" was used to define a video that had over 1 million views. Nowadays, the term is used more to identify videos with a view count of over 3 million rather than 1 million, because it is much more common for videos to get 1 million videos these days.
The term "301 club" was widely used in the 2011-2014 era of YouTube. It referred to a glitch in the way YouTube calculates views when a video is first posted. For a while the view count is stuck at the number 301, but eventually it is updated. YouTube stated they will eliminate this glitch in the near future.
1 billion views
A video had not reached 1 billion views until 2012. As of 2016, Gangnam Style by Officialpsy is the most watched video on YouTube, with over 2 billion views. However, videos are hitting 1 billion views much faster than they used to. A good example is TaylorSwiftVEVO, which has two videos with over 1 billion views, the only channel to have so.