Each "nine" of availability is x harder than the last:
% ( nines): days downtime/year. Acceptable for internal tools.
% ( nines): hours downtime/year. Standard for most web apps.
% ( nines): minutes downtime/year. Required for critical business systems.
% ( nines): minutes downtime/year. Requires redundant everything.
Moving from to nines typically requires: multi-region deployment, automated failover, no single points of failure, and major engineering effort.
In interviews, % is usually sufficient unless specified otherwise.