"Haisui-no-jin背水の陣" means fighting with your back to the water - a desperate last stand with no retreat possible.
It's about giving your all when there's no way back.
EX.
背水の陣(はいすいのじん)でこの戦い(てたかい)に挑んだ(いどんだ)。
I took on this battle with my back against the wall.