What is the safety car in F1?

After a crash the field slows down: the Safety Car bunches everyone up, the Virtual Safety Car keeps the gaps, a red flag stops the race.

Safety car

A car leads the field at reduced speed and nobody may overtake. Gaps disappear, which can wipe out a big lead, and a pit stop costs about half the usual time. That is why many drivers stop as soon as it comes out.

Virtual Safety Car

Everyone slows to the same set speed on their own, without a car leading, so the gaps stay about the same. A pit stop is cheaper here too.

Restarts

When the safety car goes in, the leader sets the pace and overtaking is allowed again from the control line.

Why do some drivers not pit under the safety car?

Staying out keeps track position. If they already stopped or are on fresh tyres, giving up places is not worth it.