This verse refers to a plan devised by the elders of Israel to secure wives for the remaining men of the tribe of Benjamin after a near-extermination of the tribe. Let's break down the meaning:

"They commanded the children of Benjamin..." This refers to the elders of Israel who were concerned about the tribe of Benjamin going extinct and breaking their oath not to give their daughters in marriage to Benjamites (because of the Benjaminite city of Gibeah's horrific act).
"...saying, 'Go and lie in wait in the vineyards...'" This is the specific instruction they gave to the Benjamites. The elders were advising the Benjamites to hide in the vineyards near the city of Shiloh. This was a strategic ambush point.

Context and Purpose

The following verses clarify the purpose. The elders knew that the young women of Shiloh would come out to dance during a yearly festival (Judges 21:21). The Benjamites were instructed to:

1. Wait in ambush: Hide in the vineyards.
2. Kidnap the women: When the women of Shiloh came out to dance, each man was to seize one of them and take her to the land of Benjamin as his wife. (Judges 21:21)

The elders provided a justification for this action: they promised to smooth things over with the fathers and brothers of the kidnapped women so there would be no retribution against the Benjamites (Judges 21:22).

In summary, this verse describes a plan for the Benjamite men to abduct wives from Shiloh by ambushing them while they were dancing in the vineyards. This was a desperate measure taken to ensure the survival of the Benjamite tribe while seemingly honoring their oath.

It's important to understand that this event is presented within a historical narrative, and many biblical scholars view it as a troubling example of the violence and moral compromises that occurred during this chaotic period in Israel's history. It's not presented as something to be emulated.