"wave-particle duality" is a slightly vague term; it makes sense though if you consider that quantum particles are probabilistic.
the way to understand it is that an electron only exists in a defined place when it is being observed, and when it isn't, it is defined only by the probability of its being observed in every place in which it can exist. we say "wave" because the probability, or "wavefunction," changes like a wave moves, and we say "particle" because the electron acts like a particle when it is being observed.
as feynman put it, an electron is not a physical object like a brick, but a concept like "the inside of a brick."
