What is phase shift?
Phase shift is often confused with time delay, although related these are two different quantities. Phase shift specifically refers to the rotation of a complex number. For example given the complex number $x = Ae^{j\theta}$ which through Euler's formula has real and imaginary components given as $A\cos(\theta)+jA\sin(\theta) = I + jQ$, a phase shift of $\phi$ would rotate $x$ according to:
$$ x e^{j\phi}= Ae^{j(\theta+\phi)}$$
Resulting in real and imaginary components as:
$$xe^{j\phi}= A\cos(\theta+\phi)+jA\sin(\theta+\phi)$$

Time domain and frequency domain waveforms can be represented as complex numbers where each sample has a magnitude and a phase. A phase shift would be a rotation of those samples, in either domain (so we can have a phase shift in time, or a phase shift in frequency). A constant delay in time has the property of a linear phase in frequency: the phase shift versus frequency will be linearly increasing in the negative direction for a fixed delay in time. For instance, given the observation of a sinusoidal tone at the input and output of a cable (fixed delay), for an extremely low frequency we would see the result of a very small phase difference between the sinusoid measured at the output compared to the input, but as the frequency is increased, this measured phase would increasingly larger in direct proportion to the increase in frequency.
For further related details, see:
What is an Intuitive Explanation of the Phase of a Signal
How does a shift in time domain result in phase shift in frequency spectrum?
Meaning of Phase response of a filter? In simple words?