Using speed test sites like speedtest.net will not give you an accurate result as these only measure the aggregate bandwidth of your internet connection, not the bandwidth to a specific server over a single connection. As internet routes change, you may find the server with the best bandwidth changes over time.
The program requires your Twitch stream key to be able to initiate a connection. Your stream will not go live during the test.
The TCP Window lets you experiment with different TCP settings. The default "Automatic (OBS)" mode replicates the mode used by Open Broadcaster Software. Other streaming software may use the System Default. Generally the higher the bandwidth and latency between you and the server, the bigger the TCP window needs to be for maximum throughput. For basic testing just leave this as it is.
Testing can take a while so you can choose which regions you want to test.
Bandwidth should be obvious. The test is capped at 10mbps, so you may see 10000kbps+ for some servers.
RTT (round trip time) is the time it took to establish the connection. Generally the lower the latency to the server, the more stable the connection will be.
Quality is an attempt at rating the connection quality based on the number of TCP re-transmissions and average delay between sends. Re-transmissions indicate bandwidth throttling, congestion or packet loss. This number will likely have some variation every time you run it due to network conditions, anything over 80 is probably stable enough for streaming.
If you get a missing VCRUNTIME140.DLL error then you need the VS2015 redistributable (x86).