S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology; often written as SMART) is a monitoring system for computer hard disk drives (HDD's) and solid-state drives (SSD's) to detect and report on various indicators of reliability, in the hope of anticipating failures before they occur.
When a failure is anticipated by S.M.A.R.T. the user may choose to replace the drive to avoid unexpected outage and data loss.
For more information on S.M.A.R.T please check out the wiki link here.