This means you have sent 50 snaps while in night mode. I am not certain what night mode is; if anyone knows please clarify in the comments. I assume it would be the power saving mode. Here is what it looks like:
For all the Snapchat trophies and what they mean use the link.
Here is an article explaining the Snapchat night mode on Android and iPhone 5s and 4s. If you don’t have the newest iPhone you may not be able to get this particular Snapchat trophy. I haven’t been able to keep up with the latest Snapchat updates but in late 2015 most Android users were unable to use night mode. I have an iPhone 5s and I am unable to use night mode.