Self-Monitoring, Analysis and Reporting Technology (SMART or S.M.A.R.T.) is a feature in many Hard Disk Drives (HDD) to detect when the drive is facing imminent failure. You should immediate backup the disk as soon as possible. SMART is only found in magnetic drives (not Solid State Drives, SSDs).
S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology; often written as SMART) is a monitoring system included in computer hard disk drives (HDDs) and solid-state drives (SSDs). Its primary function is to detect and report various indicators of drive reliability with the intent of anticipating imminent hardware failures.
When S.M.A.R.T. data indicates a possible imminent drive failure, software running on the host system may notify the user so preventive action can be taken to prevent data loss, and the failing drive can be replaced and data integrity maintained.