In a nutshell
When every frame is full and a new page is needed, a replacement policy decides which page to evict. FIFO drops the oldest-loaded page, LRU drops the one unused for longest, and Optimal drops the one needed farthest in the future — the unbeatable benchmark, since it can see ahead. Fewer page faults is the goal; FIFO can even get worse when given more frames (Belady's anomaly).