Understanding TM Simulator

I was just looking at the turing machine simulator code and came across the following statements

"The tape maps time and position to symbols. To calculate the symbol, we have to look at t开发者_StackOverflow中文版he machine one step earlier. If at that time, the head was at the requested position, the symbol has changed according to the table depending on the previous symbol at the same position and the state the machine was in. Otherwise, the symbol didn't change."

What does the part in italics mean? What does requested position mean in this context?

By "requested position" he means the position that you're trying to read, i.e. the value of the template argument pos given to tape.