When I submit a run above description theres this thing- splits i/o ID eg c6 for https://splits.io/c6. What does it mean?

Welcome to the community, RetroGamer!! I wish you the best of luck on your runs!

What that does is gives you the option to include your personal splits that you did for that run (a good example would be watching speedruns on Twitch. The splits are the time that it takes to complete a certain section/part of that particular game. The splits all add together to give you your full time (or you can just time it starting from the beginning, and stopping at the end. Splits are chiefly there to (a) compare your best time in a certain area of a game to times submitted by other people for that section, or (b) to try to get you to attempt to beat that time, so you can get a sweet new record in that particular area). One good example is "New Super Mario Bros Wii". We split after every level, so while we have the overall time at the end, we also have the time that it took for us to beat each individual level.

Submitting your splits is optional, but if you don't submit with video proof, it is preferred in a lot of communities that you at least have splits (you can download LiveSplit, which can help you get the individual splits all set up).

I hope that helped explain your question. :)