A G7 chord contains the notes G, B, D, F. It is built from the dominant 7th formula 1 - 3 - 5 - ♭7, with G as the root.
What chord does G7 resolve to?
C major. G7 is the V7 in the key of C, and its tension resolves home to C — the textbook dominant-to-tonic cadence that underpins an enormous amount of folk, blues and early rock.