-A String is just text. For example, "The quick brown fox jumped over the lazy dog" is a string.
-JSON is basically just a type of data that uses text. In C2 it's mostly used to store all the information about something to use at a later time, like an array, a dictionary, or an entire object's properties & state.
-Absolute value is a number's distance from 0. Positive is above 0, negative is below 0. abs() is commonly used to just convert a negative number into a positive one. Like abs(-100) turns into 100.
-Instance Variable is just a variable that belongs to an object. For example, say you make an enemy object. When you place this enemy in the a layout, it is known as an Instance. You can give the enemy a variable like "Strength", and each instance can have a different value for it.
-Save & Load are built-in actions to save or load your gameplay. I don't use them because you can't access every bit of information that has been saved. Instead I store data in arrays or dictionaries, and save them to local storage as JSON strings. Local storage is just a way for browsers to store data for later...like a bank of data I guess.
Just keep reading the manual and learn from example .capx's around the forums or on the store. You'll get it eventually