Secondary dominants are chords that do not exist in the home key, but have a dominant relationship to chords in the home key by a V-I progression. These chords are called secondary dominants, because of their dual function as they work in both their home key and the current home key.

A dominant (V) chord will often resolve to the one chord (I), because of specific notes in the chord. The dominant 7th chord better clarifies the reason we expect this V-I resolution. Secondary dominant chords function in a similar way as dominant chords. The difference is a secondary dominant resolves to a chord other than the one chord.

A chord progression can be made more interesting by adding secondary dominants. In a progression, they are often placed in the middle or at the end to transition to a new section. The song in this video is in the key of F#. A secondary dominant is used to end the chorus and lead to the bridge to change the mood and direction of the music. It is the V7/ii (D#7) that resolves to the ii (G#m). To keep the bridge moving in a different direction than the rest of the song, another secondary dominant is placed in the middle of the bridge, the V7/vi (A#7) that resolves to the vi (D#m). The dominant 7th (C#7) ends the bridge and takes us back to the chorus, creating a V-I tension-resolution.

https://youtu.be/TxMLT-yJocM