Hide your light under a bushel
Definition 1 of 1
Pro Tip 1/3Metaphorical Origin
Understand it's a metaphor from a biblical context, implying one shouldn't conceal talents. 
Although she's an excellent violinist, she often hides her light under a bushel.
Pro Tip 2/3Cultural Expression
It's a primarily British English phrase, less common in American English but still understood. 
He hides his light under a bushel, not realizing the phrase is less common here in the US.
Pro Tip 3/3Self-Effacing
It implies modesty, sometimes to a fault, where one's skills may be overlooked. 
Despite his many achievements, he tends to hide his light under a bushel.