Price = year + (difference between actual miles and average miles, x-xbar) + (year * difference between actual miles and average miles, year(x-xbar))

I am looking at this regression formula, but I don't understand what is going on. Could someone explain it a bit better to me. Not the idea of multiple regression, but why have x-xbar, and (year(x-xbar)) as independent variables?

Also, I understand how to interpret the partial regression coefficients, but what does

β2x2+β3x3 mean. Not separate, but together. Say something like β2(50)+β3(100350). How do you interpret Price when changing two x at the same time? So, x-xbar= 50, and year(x-xbar)= 2007(50)= 100350. I hope I explained myself clear enough. I've been stuck on this for some time now.