If a system has availability of 99%, it is called two nines availability as the number nine appears two times. Even a 99% available system gives almost four days of downtime a year, which is unacceptable for services like Facebook, google.
For 99.9% availability is known as three nines and 99.99% as four nines availability. Five nines availability (99.999%) gives a 6 minutes downtime in a year, which you can say is the gold standard of high availability.
High availability (HA) is a characteristic of a system that aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period.
Having high availability comes with trade-offs like higher latency or lower throughput. When you are a system designer, it’s your job to think about these trade-offs and decide how high available systems you need. Maybe a part of your system needs to be highly available, not the whole system.