There are many different densities, high and low are relative to the chipset you're using. What seems to be the problem?
For Intel BX/ZX/LX/EX/TX chipsets, the density limit is 16MB per CHIP. That means it supports a maximum of 256MB on a 16-chip module, 128MB max on an 8-chip module, etc. And it also supports lower densities.
For the Intel VX chipset it was really bad, 2MB per chip.
Most other chipsets have much higher density limits, so that you'll likely not need to ask the density question. But since you're asking, you'd better have a reason why!
In the days of the VX, anything greater than 2MB/chip was considered high-density. In the days of the BX, anything greater than 16MB/chip was considered high density, etc.