A dominant seventh chord is a seventh chord that carries the dominant function. In C major, G7 is the typical example, and it creates the tension that makes you want to return to C.
The basic idea
Placing it just before the landing point of a chord progression strengthens the sense of resolution.
What to check when you use it
Play G→G7→C and compare how the progression feels once the 7th is added.
It is one kind of seventh chord, but the important point is that it carries the dominant function.
