There has been a big push in recent years to make the sport safer for drivers, especially after the death of Jules Bianchi in 2015. In 2009, Felipe Massa missed much of the season after being hit in the head by a broken suspension part. The shield is essentially a windshield or canopy, protecting the driver from anything coming into the car. The open top allows the driver to escape quickly.
Vettel said the curvature of the shield distorted his vision and made him dizzy. He also complained about irregular airflow coming over the shield on the back straight, forcing his head forward.
We'll have to wait to see how things develop. The FIA is determined to introduce some sort of head protection, but the current solutions are proving difficult to implement.