Literally, it means : As every thread of gold is valuable, so is every moment of time.
一 寸 光 阴 一 寸 金 ， 寸 金 难 买 寸 光 阴
yī cùn guāng yīn yī cùn jīn cùn jīn nán măi cùn guāng yīn
寸
cùn
a unit of length / inch / thumb
光
guāng
light / ray / bright / only / merely / to use up
金
jīn
gold / metal / money / the Jurchen Jin dynasty (1115-1234)
难
nán
difficult / problem / difficulty / difficult / not good
一
yī
one / 1 / single / a (article)
阴
yīn
overcast (weather) / cloudy
光阴 means time
As every thread of gold is valuable, so is every moment of time.
