The Bekenstein bound sets the maximum amount of information that can be contained in a region of space/energy, and is usually referred to in the same way as computer storage density:
For example, a single hydrogen atom, if it were to code as much information as permitted by the Bekenstein Bound, would code about 4×106 bits of information, since the hydrogen atom is about one Ångström in radius, and has a mass of about 1.67×10−27 kilograms. (source)
Nature permits a surprising amount of information to be encoded before the Bekenstein bound is reached. For example, a hydrogen atom can encode about 1 Mb of information — most of a floppy disk. (source)
The "Bekenstein bound" leaves room for a million bits in a hydrogen atom (source)
But what does this really mean? How could any information be stored in a hydrogen atom?