This is the 35th most frequent Vietnamese word.
lên
"To go up," "rise," or "ascend."
Here 'lên' indicates upward movement to a higher level.
Tôi đi lên tầng hai để lấy sách.
I go up to the second floor to get the book.
Here 'lên' indicates the action of boarding a vehicle.
Anh ấy đã lên xe buýt để đi làm.
He got on the bus to go to work.
Here 'lên' is used to describe an increase in level or amount.