A trailing P/E uses the last four quarters of eps; a forward P/E uses estimates. High P/E means the market expects growth; low P/E means it expects less or sees risk. Comparing P/E across industries is mostly meaningless.
For traders, P/E is context, not a signal. Stocks with a P/E of 80 can double and stocks at 8 can halve.
Example: a stock at $150 with trailing EPS of $6 has a P/E of 25. If forward EPS is expected to be $7.50, the forward P/E is 20.
Related: eps, market-cap, earnings-report, index