That is the onion plants flower. Onions are biennials and will flower their second year of growth; they can also be tricked into thinking that they went through winter from the temperature swings in the spring. If you let it continue growing, numerous small white flowers will open.
You do not want onions to make the flower though. When an onion plant flowers, the flower grows through the bulb. This makes the onion rot faster so the onions with flowers will need to be used first. The flower won't affect the eating quality.
It is normal for a few onions to make flowers. If the majority of your onions make flowers, you didn't plant them at the right time. I planted onion plants in October from seeds that I started in September. I planted around 60 plants total from seeds and only one plant is bolting. Since they were tiny when the cold arrived, they weren't old enough to flower.