What is 40-all in tennis?

40 40 other wise known as duece or 40 all is a score, like in tennis you score like 15, 15 all (if it is a point each) then 30 , then 40 , then game. or if its 40 all (duece) and one person gets the point then its advantage to them, if they then win the next point its their game b ut if the other person wins the point its back to 40 all