The graph shows the girth of the trunk of a fir tree after different numbers of years: what does the y-intercept of the graph represent?

The y-intercept represents the initial girth of the tree.  Assuming that the X-axis represents time in this scenario (as it usually does) the y-axis, therefore, represents the girth of the tree.  The y-intercept is located where X = 0.  So if no time has passed then this must be the first measurement of the girth of the tree.