Investors always want to know what's cheap - cheap relative to the opportunity set and relative to history. Cheapness could refer to any number of things - price relative to trailing twelve months earnings, to trailing earnings over multiple years, to analyst earnings estimates, to long-run projections, or a dozen other variations based on sales, cash flows, book value, etc.
Because analyst estimates tend to be tainted for a number of different reasons (see this discussion on why), we tend to focus on price relative to trailing twelve-month sales, cash flows, and earnings