Deok-su and Sae-byeoks relationship?

I have watched the Korean and English dub of squid game in Netflix and in the first episode, a fight broke between Deok-su and Saebyok (sorry I can't spell names). Deok-su said something along the lines of "I gave you food, home, and money and you stab me in the back?". what does this mean? does anyone have any information on what has happened before? thank you!