I read this on Quora in this question in an answer by Peter Webb.
The magnetic forces are not real forces, they are virtual forces. They essentially derive from the time lag it takes an electric field to propagate, and that depends on the frame in which you take your measurement
What is exactly meant by 'the time lag it takes an electric field to propagate'?
The distinction between "real" and "virtual" forces that seems to be made here is non-standard. Both electric and magnetic fields depend on the frame of reference, and there is no evident meaning to calling one real and the other virtual.
That said, what this statement is likely trying to get at is the very correct notion that electromagnetism can be seen entirely as "just" electrostatics in the form of Coulomb's law together with the theory of special relativity, see this question and its answers. Since relativity inherently comes with limiting the speed of propagation to the speed light, one might phrase that as "essentially[...] the time lag it takes an electric field to propagate", but once again, that is a non-standard viewpoint.
