rock (or something)

Military gear is designed according to three general principles:

Make it durable enough to withstand heavy use; issued items will be used. A lot. And probably by more than one person over its lifetime. In the case of personal gear like load bearing equipment, armor, and weapons, they will be used extensively in training, drills, marches, and sometimes punishment, and still need to be in good enough shape to be used in actual military actions. Gear is expected to be rough-handled, stomped on, and maintained only to the minimum enforced standards by people who generally don't want to be maintaining it, because they'd rather be sleeping, playing cards, or staring vacantly at a crack in the ceiling.

Make it strong enough to withstand most attempts to either accidentally or deliberately break or alter it; this is much easier said than done. Typical engineering standards, depending on discipline, are to overengineer a given thing by margins of 10-25%. Mil-grade gear is often hilariously overdone. For example, a pull-out writing desk at a navigator's station on one particular airframe is stenciled, in two inch high letters, "NO SEAT" and "NO LOAD OVER 25 LBS", however, everyone understands that no matter what it says, people are going to sit on it, stand on it, use it as an anvil, etc. and so it's actually capable of supporting at least 275 lbs for extended durations. This is part of the reason that "mil-spec" has become another useless buzzword, much like "tactical" or "all natural".

Make the instructions simple enough that a child could understand them; instructions accompanying (or most often, printed directly onto) military gear are presented in such a way that a five year old could understand and execute them, given enough physical strength or dexterity. Directions that are accompanied by inline, self-explanatory diagrams are referred to as "Army proof", because you can understand how to operate the piece of equipment without having to be able to read.

The culmination of this last philosophy is found in the phrase "rock or something". Immortalized by the Army proof instructions on the water-activated heater found in MREs, the user is instructed to prop up the activated heater, open end up, with aforementioned "rock (or something)". The accompanying illustration has a picture of the heater propped up on a rock, and the rock itself has the words "rock or something" written on it.

They were afraid that, in the event that no rocks were available, we would resort to eating cold rations, rather than disobey the letter of the instruction.