Step 1 (Everyday picture): Imagine the electrons sitting inside a shallow well at the metal surface. The work function is the depth of that well, that is, the energy price an electron must pay to climb out and become a free photoelectron.
Step 2 (Where it appears): Einstein's photoelectric equation \(h\nu = W_0 + K_{max}\) shows that of the photon energy \(h\nu\), a fixed part \(W_0\) is spent just in liberating the electron, and only the surplus appears as the maximum kinetic energy \(K_{max}\).
Step 3 (Material property): Its value is fixed for a given surface (for example about 2 eV for caesium and higher for metals like platinum). A smaller work function means electrons are emitted more easily, so such metals are used in photocells.
\[\boxed{W_0 = \text{smallest energy needed to remove an electron from the surface.}}\]