It is the value of the radius of the ground state electron orbit in atomic hydrogen.

它就代表氢原子基态电子,的轨道半径。

麻省理工公开课 - 固态化学导论课程节选

That is the electron in its lowest orbit, to the nucleus of atomic hydrogen.

那就是氢原子原子核外电子，最低轨道到情况。

麻省理工公开课 - 固态化学导论课程节选