Nah. A substring is the same for every language, and, for Playmaker.
For any programming language, as you might already know, a substring is a portion of a string, being itself a string, and a string is a char array.
The "Start Index" value is the index where you want to start to catch the substring. Have the string "UnitedNations", as a string. I want to extract from there the "Nations" substring. As it's a string itself, I'll want to store it in a string. Then, as a zero-indexed array, the "N" for "Nations" wil be at position 6 (because it's the 7th char), and you'll define the length of your substring in the other field.
As, for the other actions, it might differ from Get String Left, but would work just as the same as Right. The main difference is that Get String Right will start from the char at position 0, and Get String Left will start from the last char, whilst you can have Get Substring starting at any position within the string.
Hope that helps.