Complex sentence: Adding a minor seventh to a major triad creates a "dominant" seventh (denoted V7), which are often the dominant chords in three-chord progressions, in which they increase the tension with the tonic "already inherent in the dominant triad".
Split into simpler sentences: Adding a minor seventh to a major triad creates a "dominant" seventh (denoted V7). Dominant sevenths are often the dominant chords in three-chord progressions, in which they increase the tension with the tonic "already inherent in the dominant triad".