According to the Bohr model of the atom, the single electron of a hydrogen atom circles the nucleus in a specific, defined orbit or energy level.
The electron moves around the nucleus in a circular path, called an orbit, at a specific distance from the nucleus, this distance is called the radius. The energy of an electron in an orbit is related to its distance from the nucleus, the closer the electron is to the nucleus, the lower its energy, and the farther away it is, the higher its energy.
The Bohr model is an early model of the hydrogen atom, it explains the hydrogen spectrum and the stability of the atom but it's not accurate for explaining the behavior of atoms with more than one electron. It's been superseded by the more accurate and general quantum mechanical models like Schrodinger equation.