This is the 8693rd most frequent Russian word.
парус
Sail.
In this sentence, 'парус' is used to signify a sail of a ship or boat.
На горизонте я увидел белый парус.
On the horizon, I saw a white sail.
Here, 'парусом' is the instrumental case of 'парус', indicating what the action 'шел' (sailed) was performed with.
Корабль шел под одним парусом.
The ship sailed under one sail.
'Паруса' is the plural form of 'парус', used to indicate multiple sails.