Page Life Expectancy is a Perfmon counter that’s supposed to track how long pages stay in memory before they get flushed out to make room for other pages that are needed instead.
Paul Randal has blogged this a few times, and rather than rehash ’em, I’d rather point you to a couple of his roundups:
Knee-Jerk PerfMon Counters: Page Life Expectancy Page Life Expectancy isn’t what you think
And layer in a few of my own observations for context:
Page Life Expectancy goes up 1 for each second that you’re not under memory pressure. Restart your SQL Server instance and