Post-money valuation reflects the company's worth after the round closes and the new investors' shares are issued. It's the basis for calculating new-investor ownership: investor's percentage = investment ÷ post-money.
Post-money is more useful than pre-money for understanding the actual dilution math and for comparing rounds. A $10M raise at $50M pre-money is identical to a $10M raise at $60M post-money, but the framing emphasizes different things.