百 (bǎi): Hundred. In this context, it's used poetically to mean “many” or “a lifetime.”
年 (nián): Year.
好 (hǎo): Good, fine, well. Here it carries the strong connotation of harmony, love, and affection.
合 (hé): To unite, to join, to combine. It signifies the union of the couple.
Together, 百 (hundred) + 年 (year) + 好 (good/harmonious) + 合 (union) literally form “hundred years good union.” This paints a beautiful and clear picture of the intended blessing: a long and harmonious life together as a married couple.