The Pressure Stall Information (PSI) metric in Linux indicates where a database system's workload is being slowed down by CPU, RAM, or I/O. The metric can be accessed through the `/proc/pressure` filesystem and can be queried from SQL using the file_fdw extension in PostgreSQL. To enable PSI, the kernel version must be higher than 4.20, and it must be enabled at boot time. Once enabled, PSI provides valuable information about system bottlenecks, which can help database administrators and developers optimize their systems for better performance. By monitoring PSI metrics such as CPU, I/O, and RAM pressure, users can identify areas of the system that require scaling or optimization to improve overall throughput.