Literally: snow – middle – pine – Oriental arborvitae tree
Alternately: Not swerving from one’s intent, principles, or loyalties despite external situations. Not bending to circumstances, pressure, or fads. Most trees drop their leaves in the fall and remain bare until the weather warms in the spring. But trees such as pines or the the “Oriental arborvitae” (an evergreen native to mainland East Asia but not to Japan) don’t change in the face of changing circumstances.
Notes: This is a pretty obscure phrase – most of my usual sources don’t have it – and I haven’t found any citations for possible origins, although the combination 松柏 is attested from at least 751 CE in the 懐風藻 (Kaifuusou), a collection of Chinese-style poetry written in Japan.