Dated, somewhat literary expression for something good or desirable. Often used in set phrases or poetic contexts.
In old tales, there is a saying that 'good things come from afar.'
He kept seeking good things.
Alternate spelling using the kanji 物 for 'thing'.
From the classical adjective 良し (yoshi, 'good') in its attributive form 良き (yoki) + もの (mono, 'thing'). The form 良き is now archaic outside of set expressions.