The exponential function models the population of Winnemucca Nevada.

Where
t is the number of years since 1990
P is the population size at "t" years
1990 is the starting time (t=0), the population at that time is the initial value of the exponential function.
An exponential function has the following structure:

Where
x is the independent variable (time)
a is the initial value
b is the growth factor
So the initial value of the function is the value that multiplies the growth factor.
For the given function, the initial value is a=6191
So the population in 1990 was 6191