This is the 8942nd most frequent Russian word.
рос
"Рос" - "grew" (past tense of "to grow" for he).
Here, 'рос' is the past tense of the verb 'расти' (to grow), used to describe personal growth or upbringing.
Я рос в маленьком городке.
I grew up in a small town.
In this sentence, 'росли' is the plural past tense of 'расти' referring to branches on a tree.
У дерева росли густые ветки.
The tree had thick branches growing.
Here, 'рос' describes the rapid physical growth of a child.