R bquote Function Explained

Cited from An R bquote example

When a plot is annotated with mathematics symbols in R, the use of expression may be required.

For example,
text(0, height[2], labels=expression(Y[med] ~ "=" ~ B*x^2), cex=3).

Contents surrounded by square ("[" and "]") brackets appear in subscript.

The tilde "~" operates as a separator, and does not show up in a plot.

If we wish to introduce variables in the annotation along with the mathematics symbols, the bquote function may be used.

For example,
text(0, height[i], labels=bquote(Y[.(z2[i])] ~ "=" ~ .(z1[i])*x^2), cex=3)

.(variable_name) retrieves the value stored in the variable and place the value inside the expression.