Definition
Until death separates us; a common phrase stated between the bride and the groom at a Christian wedding, indicating a commitment to their union.
Origins
Originates from the 1549 Book of Common Prayer, with the original wording being "till death us depart" (using the obsolete “to separate, part” sense of depart).
How People Actually Use It
a phrase that, apparently most people don't actually mean when they get married
"did you know that 40-50% of marriages end in divorce? I guess "till death do us part" means nothing anymore"
Source: Urban Dictionary