The Greek adjective barys means heavy, weighty, grievous, oppressive, or burdensome. It is the antonym of elaphros (light). Used literally of physical weight and metaphorically of the severity of burdens, demands, commandments, or accusations. The noun baros (weight, burden) derives from the same root.
Jesus used the concept of barys in Matthew 23:4 — the Pharisees tied up heavy burdens and laid them on people's shoulders. In contrast, Jesus declares His yoke is easy and His burden light (Matthew 11:30). The contrast is theologically crucial: religious legalism creates crushing weight; grace liberates. John also uses barys positively in 1 John 5:3 — God's commandments are "not burdensome" to those who love Him. Love transforms what was weight into delight.