It could mean various things. Maybe it was a strategic decision due to financial reasons. CNN might have found that Great Big Story wasn't generating enough revenue or fitting well into their overall business model. Another possibility is that it had editorial or content - related issues that made it no longer viable within the CNN framework.
One possible implication is that the employees working on 'Great Big Story' may lose their jobs. Another is that the viewers who were interested in this type of content will have to look elsewhere for similar stories.
When we see 'cnn shutting down great big story', it implies that CNN is putting an end to this specific offering. CNN might have evaluated the performance of 'Great Big Story' in terms of audience engagement, revenue generation, and its fit within the overall brand. If it wasn't performing well in any of these aspects, for example, if it wasn't attracting enough advertisers or if the content wasn't resonating with a wide enough audience, then shutting it down would be a strategic decision to cut losses and focus on more successful ventures.
It could mean that CNN has decided to stop the operations related to Great Big Story. Maybe it was not meeting their strategic goals, or there were financial reasons behind it.