In terms of intervals, a fifth is an interval formed between two notes with five whole tones separating them. For instance, if the note C is the starting point, counting five whole tones upwards (C-D-E-F-G), the fifth note is G, which is the perfect fifth above C.
In general, the fifth has a prominent and harmonious relationship to the tonic (do), creating a sense of stability, resonance, and musical resolution in many musical contexts. It is considered one of the essential building blocks of harmonic progressions and is commonly found in chord structures, melodies, and cadences throughout different musical genres and styles.