Welcome! hopefully you grow to love this community as much as I have!
reset means ending a combo, and starting a new one. generally the second combo that is started is inescapable, but there are a few that are mixup related.
50/50 - when your opponent has to guess whether your going to hit them overhead or with a low.
Fuzzy guard - when you block high n low consecutively, and your character does not stand up/duck and just blocks the attack. usually done by releasing down/up
MK uses a number system to communicate inputs. for example. F4 would be Forward + B or circle button depending on the system you are on. 1 = X,square 2 = Y,Triangle 3 = A,X 4 = b, Circle these are default settings, it will change depending, but I dont know which is LP,HP w/e..ill let someone else handle that lol
Hope this helps!