A radial node is a spherical shell (i.e. hollow sphere) where the wave function has a value of zero or is otherwise changing signs. It is called “radial” because the points of zero possibility are a fixed radial distance from the center.
For a given principal quantum number and azimuthal quantum number , there are radial nodes in this subshell.
Here’s how a spherical node looks like on a cross-section of the wave function:
