Engrams are much like a memory capsule, much like an airplane's black box.
They are recorded with a human yet mechanical recording device, the reptilian brain, that never sleeps.
The content of the engrams is every detail around a traumatic incident where you were in an altered state, sleeping, fainted, in anesthesia, or in a shock. None or little of it is available to the conscious mind, directly, instead the engrams act like a hidden program, like an ad-ware, that pops up whenever there is any similarity between the present moment and the moment of the engram recording.
The contents of the engram make you do weird and unreasonable stuff, act out the engram, or dramatize... more about this later