What does initial value mean? (the topic matter is linear functions)

Initial value can mean many things, but what I believe it means in this area is the value that you start with (obviously), so in the equation (y=mx+b), the initial value is the b. It is where the line intercepts the y-axis, and that is your initial value, I believe!
I hope this was helpful and answered your inquiry! If you have any further doubts or questions, please let me know so that I can help you!