Something that's sobering is serious, and even a little bit worrying. It's sobering to learn that someone you love is very ill.
You can use the adjective sobering when you talk about news that feels heavy and important. Environmental studies about climate change are sobering, and statistics about the number of stray dogs and cats in the country are sobering too. Things that are sobering make you feel sober, or serious and solemn. The Latin root, sobrius, means "not intoxicated," which is also the original definition of sober.