An interval of a perfect fourth is the distance from the first degree (or the root) of a scale to the fourth degree.
For example, C to F is a perfect fourth. C is the first (or root) note/scale degree and F is the fourth note/scale degree. The first two notes of "Here Comes The Bride" are a perfect fourth.
Harmony with stacked 4ths became a signature component of modal jazz in compositions like "So What" (Miles Davis) and "Maiden Voyage" (Herbie Hancock.) 4ths are also common in jazz melodies of the era, as heard in the melody to Eddie Harris's "Freedom Jazz Dance."
In neo soul guitar, 4ths can be a useful tool to harmonize the pentatonic scale.
To learn more, see "Perfect 4ths"