pitää kynttiläänsä vakan alla

From the Bible, literally pitää (“to hold”) kynttilää (“candle”) +‎ -nsä (“third person possessive suffix”) vakan (“of the bushel”) alla (“under”); "to keep one's candle under the bushel".

pitää kynttiläänsä vakan alla

1. (idiomatic) to hide one's light under a bushel (to conceal one's talents or positive qualities, especially due to modesty or shyness; to avoid attention)

   Älä pidä kynttilääsi vakan alla!

   Don't hide your light under a bushel!

For the conjugation of the verb, see pitää, kynttilää takes a possessive suffix according to the subject of the sentence, vakan alla is not inflected.

• olla turhan vaatimaton