Normally, one would have a certain pattern and they would make an equation to make the prediction for what values would come next in a sequence ( or at some x value ), but if one had an equation such as y=3x+4 ( I am using examples because this was a very general question ) they could see that when x is 0 y=4 and that the y value increases three units for every change in x value. since y=4 when x=0, 4 may be called the initial value as well as the y-intercept ( in fact this is generally true for all mathematical models ). This is where the pattern starts.