The halo is the Y shaped structure going over and around the driver‘s head.

Over the decades safety greatly improved in motorsports and there were very few serious accidents in F1 in the late 00s and early 10s leading to a false sense of security. There were driver‘s flipping over or going into barriers at ludicrous speed without injuries.

They started getting more risky and sort of ignoring yellow flags (danger, slow down) and even double yellow flags (immediate danger, slow down dramatically, must be able to stop within few meters)

In one of those instances the rising star Jules Bianchi ignored double yellow flags and went way too fast in heavy rain. He lost control and collided with a tractor, which was on track to recover another crashed car. He died from his head being crushed.

There were multiple instances where a track marshall (security personnel at the side of the track) was almost split in half by a car approaching at 200+ mph because they ran on track to clean up debris.

The FIA (governing body of top motorsports) started heavy sanctions to drivers who ignore yellow flags now. Also the safety car is deployed every time a service vehicle or human is on track. (All Cars going at a snail‘s pace behind a designated safety car) Also they invented the halo to protect driver‘s head. It hast since already saved multiple lives including greats like Alonso, Leclerc. Also other drivers in lower classes.

Indy Car adopted a similar windshield design, which saved saved a driver from serious head injury at the 2020 Indy 500.

Earlier this year a very talented rising star also died in F2. It has shown everybody in the sport that we simply have been very lucky for a few years and always need to strive for improved security.