The pre-money valuation is what you are worth before the investment. So you're saying "we have a company worth $10 million" and the investor is saying "cool, le

The pre-money valuation is what you are worth before the investment. So you're saying "we have a company worth $10 million" and the investor is saying "cool, let me give you $5 million." So now you have $10 million in company plus $5 million in cash, so you are worth $15 million (that's called the post-money valuation.) The investor gets $5m / $15m—33% of the company.