A node represents a region in space where the value of the wave function (Ψ) is zero.
From the wave equation, we know that the magnitude of the wave function helps us identify the location of an electron in space. This space as we also know is called the atomic orbital. Nodes are therefore such points in the orbital where the possibility of finding an electron is zero i.e. the magnitude of the wave function is zero and electron density is zero. Imagine this as the point where a wave has zero amplitude.