The idea is that if you promise to drop nuclear weapons on any country which attacks your capital, Russian tanks will not attack your capital. Whether this is a wise strategy is unclear, but countries like Pakistan[1] and Israel[2] have employed such a policy, and none of their unfriendly neighbors have yet managed to send tanks into Islamabad or Tel Aviv.
[1] https://en.wikipedia.org/wiki/Full_spectrum_deterrence
[2] https://en.wikipedia.org/wiki/Samson_Option