The buffer pool in SQL Server is an area in memory for caching data. Once data is read from disk, it can be kept in the buffer pool and SQL Server can check here for data when it needs to be found in the future. If requested data is not found in the buffer pool, a hard page fault can occur, meaning data needs retrieved from disk. It’s possible that data is found somewhere else still in memory which is called a soft page fault.
Click through for Chad’s thoughts on what a good page life expectancy looks like. My minor addition is that the number isn’t as important as the shape of the curve: if you have a fairly stable PLE above some arbitrary threshold (well above 300 seconds!), you’re probably in good shape. If your PLE sawtooths, your server’s RAM Pez dispenser needs refilled.