Their resignation to the harsh realities was a sign of maturity.

resignation 是动词 resign 的名词形式，resign 意思是"辞职、放弃"，加上后缀 -ation 表示动作或状态。在这里引申为"顺从、接受"的意思，指对无法改变的现实采取被动接受的态度。