When you refinance to a lower rate, that reduces the size of your mortgage interest deduction, which affects your savings from refinancing. The Mortgage Refinance Calculator can take that into account, which is why it asks for your income tax rate.
Look up yo in Wiktionary, the free dictionary. Yo is an English slang interjection, commonly associated with American English. It was popularized after being used commonly by the Italian-American community in Philadelphia, Pennsylvania since the 1940s.
This expression is the evolution of the phrase 'carpe diem' ('seize the day'). It is used to inspire courage and bravery, or to justify doing something idiotic and embarrassing. You will see YOLO spelled with its four letters, as well as with the pound sign hashtag '#YOLO'.