Availability is the ability of a system to remain operational and accessible to users without interruptions or downtime. It is often measured as a percentage of time that a system is operational within a given period.
For example, a system with 99.9% availability means it is operational 99.9% of the time, allowing for only 8.76 hours of downtime per year. Availability can also be measured as "number of nines". For instance, "five 9s" refers to an application that is operational 99.999% of the time.
One practical way to improve availability is by implementing redundancy in critical components of the system. For instance, using load balancers to distribute traffic across multiple servers can help prevent a single point of failure from causing downtime.
It is important to note that a system with high availability doesn't mean the application is working as expected. Availability is usually decoupled from user experience. An application can be operational but behaving in ways that do not fulfil the user expectations. You will sometimes hear engineers say "Nines don’t matter if users aren’t happy".