The slope of the line dictates the age of the sample.
During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase.
This, thus, allows a different rate of radiogenic Sr to evolve in the separate rocks and their component minerals as time progresses.
The age of a sample is determined by analysing several minerals within the sample. If these form a straight line then the samples are consistent, and the age probably reliable.
For example, consider the case of an igneous rock such as a granite that contains several major Sr-bearing minerals including plagioclase feldspar, K-feldspar, hornblende, biotite, and muscovite.
Each of these minerals has a different initial rubidium/strontium ratio dependent on their potassium content, the concentration of Rb and K in the melt and the temperature at which the minerals formed.