Another commenter mentioned this, but it's important to understand the net present value calculation, which is:

NPV = Rt / (1 + i)^t

Rt is the incoming cash flow at time t, and i is the discount rate. This is the "theoretical" way that businesses should be valued: estimate their future cash flows, then discount them back to the present to take the time value of money into account (of course, in the real world, there is a lot more that goes into the price of stocks).

The issue is that as the discount rate approaches 0, the net present value then equals the same as ALL future positive cash flows. For large, perpetually operating businesses, this can then turn into something between "a lot" and "infinity".

Again, there are lots of other things that go into actual stock prices, but our economic system really is in new territory when the discount rate approaches 0 (or heck, goes negative!)