Prefetch is a computer system technique that aims to improve the performance of memory access by fetching data from a slower storage medium, such as a hard disk drive, to a faster storage medium, such as RAM, before it is actually needed.

The purpose of prefetching is to reduce the latency or delay in accessing data. By anticipating future data needs based on patterns or algorithms, the system can proactively fetch the required data into faster memory, making it readily available when needed.

Prefetching can be applied at different levels in a computer system, including hardware and software. In hardware, processors or memory controllers may have prefetching mechanisms that automatically fetch data from lower levels of cache or main memory into higher levels of cache. This helps to minimize the time needed to access data from slower memory.

In software, programmers can use prefetching techniques to optimize memory access. For example, they can explicitly request the system to load data into memory before it is needed, based on their knowledge of the program's behavior or data access patterns. This can be particularly useful for applications that involve large data sets or frequent memory access.

Overall, prefetching is a technique used in computer systems to improve memory access performance by fetching data in advance from slower storage to faster storage. It helps to reduce latency and optimize data availability, ultimately enhancing the overall system performance.