Motion toward or away from an observer
A star can move across the sky while approaching or receding along the line of sight. Radial velocity measures the latter component, not a complete trajectory or necessarily the full speed.
Proper motion and distance supply complementary information. Gaia combines astrometry and radial velocities to investigate stellar motion in three dimensions.
References: [1]
Spectral features provide reference wavelengths
Stellar absorption lines can be compared with laboratory reference wavelengths. Doppler shifts change in opposite directions for approaching and receding motion, and repeated spectra track time-dependent components.
Measurement needs identifiable features and appropriate spectral resolution. Line width, position and strength convey different information; broadening and an overall red appearance are not the same as a coherent shift.
Reference frames and orbital interpretation
The observing platform also moves. Gaia corrects its motion relative to the Sun to report heliocentric radial velocities, making a consistent reference frame essential when comparing measurements.
Periodic radial-velocity changes can investigate stellar or planetary companions, but orbital inference requires models and other information. A line-of-sight component does not by itself reveal every orbital orientation or spatial velocity.