r the radius at which the electron revolves.
r = Ze²/4π ε0v²
For hydrogen, z =1 and we get r1 as 53 picometre ( 1pm = 10^-12 m) or 0.053 nm. This length is called the Bohr radius and is a convenient unit for measuring lengths in atomic physics. It is denoted by me as the symbol a0 (In HC Verma a different symbol is given. I am using this symbol as a convenience).
In terms of Bohr radius the second allowed radius is 4 a0 and third is 9 a0 and so on. In general nth orbit of hydrogen atom is n²a0.