Sober October (as the name also applies), is you going sober from one or multiple substances for all of October's 31 days.
Which substance(s) you decide to quit for a month is totally up to you. People do it very differently, and it is common to just be concerned about quitting alcohol and/or cannabis for one month straight. Some people also take it to more “extremes”, like for instance also quitting caffeine and/or fast foods.
CAUTION: If you are a heavy user of alcohol, quitting it from one day to another might be lethal, seek professionnel help instead!
There are a lot of different reasons to participate, maybe you want to prove to yourself that you are able to, or maybe it is something completely different. No matter what you choose, then you have at least chosen something (which is better than nothing).
Remember, change is never too late...