Rutherford gold foil experiment
Reflect means to bounce back, as in a ball bouncing off a wall. The average density of an atom is very low, so the observed reflection was startling... like throwing many billiard balls at a mass of fluffy cotton  candy and every now and then a ball comes bouncing back!
The explanation offered was that there must be some &quot;hard&quot;, dense object inside that fluff... the nucleus. Electrons are effectively spread out in their orbitals, and tend to act individually (bound by electromagnetic forces). An alpha particle, massing $\pu{4 \times 10^{9} eV}$, could not be reflected by an electron massing ~$\pu{5 \times 10^{5} eV}$ -- that would be like bouncing a baseball off a ping-pong ball.
Since both alpha particle ($\ce{He}$ nucleus) and gold nucleus are positively charged, and the gold nucleus is held together by powerful nuclear forces (so all its particles' masses are effectively combined to ~$\pu{2 \times 10^{11} eV}$), it explained the reflection. Since reflection occurred only rarely, it implied that the nucleus was a small, dense target.