Humbleness is a quality of being modest or unpretentious. Your humbleness is what keeps you from bragging about all the places you've traveled and the many languages you speak.
True humbleness is characterized by humility and a certain quietness or reserve. Showing off, exaggerating your accomplishments, and refusing to listen to other people's advice and opinions are all the opposite of humbleness. Sometimes this noun is also used to mean an ordinary or lowly state: "The humbleness of her upbringing made paying for college a struggle."