The post-money valuation is the sum of the pre-money valuation and the money raised in a given round. At the close of a round of financing, this is what your company is worth (at least on paper).
The only reason it’s worth spending time on this term at all is that it “sets the bar” for your future activities. If your post-money after your first round of financing is $4 million, you know that to achieve success, in the eyes of your investors, any future valuations will have to be well-in-excess of that amount.
Given that all this matters for is setting the bar for the next round, it does not need too much discussion.
And frankly, if you’re focused on building value in the company you should be able to mostly ignore this — but just remember that professional investors are not looking for 10% increases in valuation. They dream of 10X and 100X increases in valuation so keep that in mind when you’re figuring out how much money to raise and how much progress you should be shooting to make between rounds.
One detail to keep in mind is that it’s not always quite as clear as raising a round and then raising another at a new valuation. Many times when new money comes into a company between rounds — be it venture debt, a convertible note, or a bridge loan — it has hooks into the next round. As a service to yourself and your existing investors, make sure to keep this kind of activity in perspective as it relates to changing valuations. When money comes in from anything other than grants or customers, it most likely has implications on valuation.