Index in plot in R

I plotted the following data in R

data$cm
 [1] 55.5 68.0 65.0 67.7 70.0 66.0 60.0 62.0 52.0 50.0 62.0 16.0 15.0 17.0 16.0 17.0 17.0

Using the plot function.

plot(data$cm ,type = "l",col="red", ylab="(cm)", lwd=2, font.lab=2, main="Comparison",
     cex.axis = 1.5,cex.lab = 1.5,font=2, cex.main=1.5)

I get the following plot

but I am wondering, what does "index" means?