The figure on the left side shows the water level is at the 0.5 liter mark, ie half a liter.
1 liter = 1000 mL
0.5*1 liter = 0.5*1000 mL
0.5 liter = 500 mL
If you poured all the water into the jug on the right (without spilling), then the water level should reach the 500 mL mark. This is the small tickmark exactly halfway between the 400 and 600.