This is a very sturdily built breakout box with one simple task: convert your SDI signal to HDMI. It has a USB-C port for power as well as to connect to a host PC (more on that in a sec) - though you can also order the converter with an external PSU - as well as an SDI out to send your signal to another SDI device further downstream such as an LCD monitor or EVF.
For such an inexpensive and small device, it adds a nice feature: you can upload a 3D LUT to allow you to monitor your image on set with a specific look, to judge exposure, gamma/contrast curves etc, which not all monitors or EVFs support. However, with the latest firmware (as of Feb 21), based on forum posts, it appears several of these units are "faulty" (hopefully solved with a bugfix) and do not display the image correctly with a LUT loaded - you simply get a completely white screen. (The current workaround is to simply disable the LUT and monitor the image as-is.)