This is the 63rd most frequent Chinese word.
于
"于" means "at," "in," "on," "to," or "than," depending on context.
In this sentence, '于' acts as a preposition indicating 'at' or 'in' with regard to time.
他出生于1990年。
He was born in the year 1990.
Here, '于' is part of the phrase '在于', indicating 'lies in', showing the dependency or location of an abstract concept.
决定权在于你。
The decision lies with you.
In this idiomatic context, '于' reflects a more literary or formal register, meaning 'with regard to' or 'to' in expressing the context of the action.