There are add ons certified by Kodi. Watch a few YouTube videos on it. I remember there were several news ones such aw BBC and then ones you log into as someone mentioned with MLB TV. I also think Netflix, Hulu, etc are all official add ons. I believe there's even one for movies and TV shows in the public domain. That's pretty cool.
All other add ons are 3rd party and have no official Kodi support. Many (not all) are illegal. Essentially 2017 version of stealing cable tv.
You can also use it similar to VLC, to watch media files locally off a device or streaming over your network. Old school XBMC cut its teeth on this. A media box 10 years before moat people had one.
I still have an old oXbox with XBMC! This blew my mind in 2006! I still use it watch my 720x480p DVD rips, over component upscaled to 1080i. The old CPU can't handle newer h.264 movie files though.
Essentially, Kodi came out of the modding community so it's roots lie with hackers/modders, but they've tried to make it more legit over the years.
Try downloading it on your PC first before you start trying to side load apps onto a Fire Stick. There's a Windows version.
If rippings DVDs & BDs for personal use was illegal, I image the app MakeMKV or HandBreak would've been shutdown years ago. He constantly updates it to get around the latest BD efforts to stop ripping. They even sell DVD ripping software at MicroCenter, Fryes, etc.
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Class of '10 - A&M Undergrad & Master's Alum