In this chapter and the next, we will study tonicization, which means treating a chord other than the $\left.\text{I}\right.$ chord like a tonic by approaching it with its dominant. In diatonic harmony, the $\left.\text{V}\right.$ chord (the dominant) resolves to the $\left.\text{I}\right.$ chord (the tonic). A secondary dominant is a major triad or dominant seventh chord that resolves to (or tonicizes) a chord other than the $\left.\text{I}\right.$ chord.
You may find that you want to analyze the $\left.\text{D}^{7}\middle/\text{F}^{♯}\right.$ in the example above as a $\left.\text{II}^{6}_{5}\right.$ instead of a $\left.\text{V}^{6}_{5}\middle/\text{V}\right.$ (which we pronounce as “$\left.\text{V}^{6}_{5}\right.$ of $\left.\text{V}\right.$”), and the $\left.\text{E}^{7}\middle/\text{G}^{♯}\right.$ as a $\left.\text{III}^{6}_{5}\right.$ instead of $\left.\text{V}^{6}_{5}\middle/\text{vi}\right.$ (“$\left.\text{V}^{6}_{5}\right.$ of $\left.\text{vi}\right.$”). Notice, however, that a $\left.\text{ii}\right.$ chord is typically minor in a major key and diminished in a minor key ($\left.\text{ii}^{\circ}{}\right.$), making uppercase $\left.\text{II}\right.$ a chromatic harmony for which the proper label is $\left.\text{V}\middle/\text{V}\right.$.
While labeling $\left.\text{D}^{7}\right.$ as $\left.\text{II}^{7}\right.$ in C major makes the root clear, it does not communicate the function of the $\left.\text{D}^{7}\right.$, which is to progress to a G major chord (the $\left.\text{V}\right.$ chord, or the dominant in C major).
Also, notice that the $\left.\text{vii}^{\circ}{}\right.$ is not tonicized with its secondary dominant in the example above. Listen to the following example to understand why diminished chords such as $\left.\text{vii}^{\circ}{}\right.$ and $\left.\text{ii}^{\circ}{}\right.$ in minor are not tonicized.