In practice, this applies to having money now vs having it in one year's time; and the basic discount rate is the interest rate you can get in a risk-free (or as close to it as possible) investment, like a bank deposit or a government bond.
Then it is used to identify the cost of borrowing, i.e. spending money now to get something (what you really want) in the future. In the long run, we're all dead. John Maynard Keynes
Otherwise - what exactly is it that you have now that's more valuable than having it later? And does that idea make sense between generations?
Surely the situation is that in real terms, ignoring ecological effects means that you're borrowing from future GDP today? And that cutting down that borrowing will lead to lower losses in the future?
Putting a figure on it and worrying about a few percentage points of difference seems academic in the context of potential flood, famine, drought, and a vastly different energy landscape.
Having 100 today is the same as having 104 in one year (with a 4% disocunt rate) - and that's what your 100 will become if you do 'nothing' with them, i.e. keep them in risk free government bonds.
So if you are told that if you do nothing, you'll have a loss of 104 from global climate change in one year's time, this is equivalent to a loss of 100 today, and if you can prevent that loss by spending less than 100 today, then you come out better than if you did nothing.
Same thing if you invest: if you spend 100 today, you want to get more than 104 out of the process in a year's time, or you might as well buy government bonds.
Then you need to incorporate concepts like inflation, the price of risk, and the cost of borrowing for you (and more items) to the picture, but essentially, the discount rate allows you to express value in the future in terms of today's money, and thus to compare outcomes in terms that are useful to decide (or not) to spend money today to change these outcomes. In the long run, we're all dead. John Maynard Keynes
