Two sinusoids at the same carrier frequency but $90^\circ$ apart are orthogonal over an integer number of carrier periods:

$$\cos(2\pi f_ct),\qquad \sin(2\pi f_ct).$$

A passband signal can therefore be written as

$$s(t)=I(t)\cos(2\pi f_ct)-Q(t)\sin(2\pi f_ct),$$

where $I(t)$ is the in-phase component and $Q(t)$ is the quadrature component.

The pair can be represented compactly by the complex baseband signal

$$u(t)=I(t)+iQ(t).$$

This does not mean the physical voltage is complex. The complex notation stores the amplitudes of two real orthogonal carrier components in one mathematical object.

Changing $I$ and $Q$ changes the carrier's amplitude and phase. Digital modulation schemes exploit this by choosing a finite set of allowed $(I,Q)$ pairs, while receivers recover the two components by correlating with synchronized cosine and sine references.