This verse is the beginning of a riddle or thought experiment posed to Jesus by the Sadducees, who did not believe in the resurrection. It sets up a scenario based on the ancient Jewish law of levirate marriage.

Here's a breakdown:

"Now there were with us seven brothers..." This establishes the players in the scenario.
"The first married and died, and having no offspring..." This is the critical point. The man died without having a child.
"...left his wife to his brother." This refers to the law of levirate marriage (Deuteronomy 25:5-10). The purpose of this law was to continue the deceased brother's lineage and ensure that his name and inheritance wouldn't be lost. If a man died without a son, his brother was obligated to marry the widow and produce an heir who would be considered the dead brother's son.

In essence, the verse describes the first step in a hypothetical situation where a woman is married to seven brothers, one after another, each dying without fathering a child. The point of the Sadducees' question, which follows, is to challenge Jesus's belief in the resurrection by asking whose wife the woman would be in the afterlife, given that she was married to all seven brothers. The Sadducees thought this posed an absurd challenge to the concept of resurrection.