phase shift
Two waves of the same frequency can be in step or out of step, one cresting while the other is still climbing. Phase shift measures that timing offset: how far one wave is delayed or advanced relative to another, like two runners on the same circular track who are not side by side.
Phase is measured in degrees or radians of a cycle, where 360 degrees (2 times pi radians) is one full cycle. A 90-degree shift is a quarter-cycle offset. Reactive parts create phase shift: in a capacitor the current leads the voltage by 90 degrees (current flows hardest when voltage is changing fastest, at the zero-crossings); in an inductor the current lags by 90 degrees. A simple RC low-pass shifts the output somewhere between 0 and 90 degrees depending on frequency.
Phase shift is why reactance and impedance need more than a single number, why filters delay signals, and why too much phase shift around a feedback loop can turn an amplifier into an unwanted oscillator. Caveat: phase only has meaning between two signals of the same frequency (or a signal and a chosen reference). Asking for the phase of a lone signal in isolation is meaningless without a reference.
Feed a 1 kHz sine into an RC low-pass at its cutoff: the output is half-strength and lagging the input by 45 degrees, a one-eighth-cycle delay.
A filter both attenuates and delays.
Phase is only meaningful between same-frequency signals; the phase of one isolated signal means nothing without a reference.