The elements in the string (the pink text within the quotation marks) is what is actually displayed on screen. Each element in the string gets the text that it should display from the arguments that come after it. So the first element (%3d) displays whatever the first argument is (@pokemon.hp, in this case), and the the second element of the string (another %3d) displays whatever the second argument is (@pokemon.totalhp). So what ends up being displayed (using Waspear from the image as an example), is "62/ 90".
The following arguments in this line of code (the ones after the first closed parenthesis) determine the position of the text, and its color. The first argument (320, in this case) determines its X axis coordinats, the second determines its Y axis coordinates, and the third number determines how the text is aligned on screen (0 = to the left, 1 = to the right, 2 = centered). Then the final two arguments determine the text's base and shadow colors.
Hopefully you can figure everything out from that.