Post-money valuation is the pre-money valuation plus the new money invested. If a company is valued at 900 million dollars before a round and raises 100 million, the post-money valuation is 1 billion dollars. An investor who puts in 100 million owns 10 percent, which is the investment divided by the post-money figure.
The distinction matters because the same headline number can mean different things. A company that announces a 1 billion dollar valuation might mean either before or after the new cash, and the ownership each investor receives depends on which.
It is also not a market price. For a private company, valuation is a figure negotiated between the company and investors, often tied to the terms of the shares being sold. Different share classes may carry different rights, so the same valuation can hide very different deals. Convertible notes and SAFEs add further complexity.
Ask whether a valuation is pre- or post-money, and work out the implied ownership for each party.