To help with some explanations:
In 90% of cases, Skryba is correct on his definition of conversion. It means “how good are you at taking random, stray hits that you were not expecting to hit, and converting them into actual combos for real damage?” It generally means the player has good game sense, because he knows what his character’s options are, even in unlikely situations that happen quickly and suddenly. He doesn’t freeze and panic, instead he is prepared to handle the situation. For the other 10%, you can use the word to describe how much damage you get after landing a “regular”, expected hit that you were planning for. A person who has poor conversions might do short, weak combos after landing a hit and not converting on his openings well.
Parry is definitely not a universal mechanic in all games… in fact, very few games have parries. SF3 3rd Strike is the most famous example, but as far as I know, Guilty Gear is the only modern tournament fighter with a universal parry mechanic. Some games (including KI) have one or two characters that can parry under certain conditions (in SFV, Ryu can parry, for instance).
Combo assist is a mode that lets you cancel normals into specials without doing a quarter circle joystick motion. Instead, you simply tap your normal, then tap another button and a predetermined special of that strength will come out. You typically have one special for punches and one for kicks, so if you want to do a special move that isn’t one of your predetermined ones, you will still have to do the motion regularly. What CAM lets you do, though, is have fun with the combo system more easily, because you can tap buttons back and forth and execute combos without any joystick motions at all. For players who really struggle with doing joystick motions, this can let them have a lot of fun with the game more quickly.
Note that CAM is not an “easy mode” that prevents you from playing the game the “proper way”. Everything you can do in a regular match, you can do with CAM turned on. If you are looking to play multiple fighting games long term, I wouldn’t recommend you get too attached to using CAM (because being comfortable with joystick motions is something you need to learn), but as a starting point I think it’s totally fine and you shouldn’t feel bad for using it while you get used to the rest of the game’s rules.
I wrote a website that helps people learn the game, as well! Check it out if you’re interested: http://ki.infil.net