Two voices a perfect fifth apart, both stepping the same way to another perfect fifth. Move the notes, watch the app catch them, and hear why the rule exists.
When two voices sit a perfect fifth apart and both move in the same direction to land on another perfect fifth, that’s a pair of parallel fifths. Say the lower voice goes C→D and the upper goes G→A: both rose a step, and C–G and D–A are both perfect fifths.
The catch is that the two lines briefly stop sounding like two people and start sounding like one thick voice — the independence that part-writing depends on collapses. The same applies to parallel octaves.
From the Renaissance through the classical era, independent voice-leading was the whole craft, so parallel fifths and octaves were avoided: they undo the illusion of separate lines. Fixing one usually means moving a voice the other way — contrary motion keeps the parts distinct.
Elsewhere the “fault” is a feature. A rock power chord is literally a root and fifth shifted around in parallel; blues and metal riffs lean on it for weight; and parallel fifths give film and folk music an open, ancient character — the sound of medieval organum. Same intervals, opposite verdict, depending on the style.