I just found a great technical description ‑see below- of how we remember in the Brain Backgrounders at The Society for Neuroscience website.
If you are looking for a less technical explanation, you will enjoy reading instead:
At first the activity of various chemicals provides knowledge with a temporary occupancy in the brain, lasting for a few minutes. The memory is in its “short-term” phase. This fresh information is evicted and forgotten unless essential molecules and genes are activated.
Many researchers believe that the process of transforming a short-term memory into a long-term memory begins when brain cells receive signals that induce reactions involving the molecule, protein kinase A. This, in turn, sets off another molecule in the cell known as cyclic AMP-response element binding protein (CREB). CREB activates genes, which are segments of the cell’s deoxyribonucleic acid (DNA). Genes hold sequences of coding molecules that provide the biological instructions for producing proteins. The development and function of the body and brain is directed by many different proteins. The genes activated by CREB lead to the production of special proteins that change the structure and activity of nerve cells. These reactions fasten information for days, weeks or longer.
The core molecular switch appears to be involved in securing the memories of facts and events, known as explicit memories, as well as implicit memories. Implicit memories remind you how to do something. They involve motor skills and perceptual strategies. The answers to the history test questions rely on explicit memories. Implicit memories remind you how to actually write the responses.
While at least some of the chemical reactions needed to convert a short-term memory into a long-term memory appear to be the same, the memory processing occurs in different brain areas. Explicit memories require the brain regions within the temporal lobe of the cerebral cortex including the hippocampus. Implicit memories are primed in the specific sensory and motor systems that are recruited for whatever the particular task is.
Thinking of all those fresh memories being “evicted and forgotten” unless a whole chain of molecular events takes place makes me wonder if we should stop worrying about why we forget and instead wonder why we ever remember!