From BiblePortal Wikipedia

Vine's Expository Dictionary of NT Words [1]

1: Πρασιά (Strong'S #4237 — Noun Feminine — prasia — pras-ee-ah' )

"a garden bed or plot" (probably from prason, "a leek"), is used metaphorically in  Mark 6:40 of "ranks" of persons arranged in orderly groups.

International Standard Bible Encyclopedia [2]

raṇks ( πρασιά , prasiá , "a square plot of ground," "a garden-bed"): "They sat down in ranks" (  Mark 6:40 ); the several reclining ranks formed, as it were, separate plots or "garden-beds."