one theory is that memories are stored as distributed associative networks of neurons and that the act of forming memories is what happens when connections are formed and strengthened. when you learn something you repeat it often enough that you "know" it. apart from the actual facts of the "knowing" there is also the same sort of distributed associative network of neurons and connections (which may or many not be the same ones) which index the knowledge. this index may be a part of your consciousness or subconsciousness and it may be less permanent or more easily damaged because it is not as big, as distributed and as strong as the memories themselves.
then think about what consciousness might be and to me i think it is also a learned pattern which is tied into the memories directly or through the indexes, but also to many other things our brains do (hear, taste, smell, see, coordinate, move, ponder, etc.). so when you have this sense of a word being on the tip of your tongue it is the conscious distributed pattern being so strong on top that it can't get through the associative network or the index well enough to get the answer. the key there is to stop trying immediately and go do something else.
numerous times when trying to debug a program and not able to figure it out (or solve some other problem) i've finally given up trying to find the answer and in the old days i was using what was called a remote batch station which we could check out overnight the doors were locked and other people could not get in, but then when you left and that door locked behind you you could also not get back in. the number of times the answer would come to me in the moments after walking out that door were enough that i learned how to take breaks and work on completely different problems or even different homework or subjects. *click* *ah ha!* many hundreds of times over many years.
the other thing that you hear is that people say to sleep on it. that is also a good method. if i have a very hard subject to learn i study it right before going to sleep. the brain works on it all night. 21 days later i'm a genius! only for a while. my memory is attrocious...

in order to keep my chops in some topics i have to restudy them regularly. like once in a while i have to relearn how to spell some words. i still have to look up "seperate" way too often <-- yes it really should be spelled "separate"... only one example of many for my scrambled noodles... aka brain...