I am introducing a tough concept early here so that it does not hit you so hard when you read about it further on.
Many memory techniques (also known as 'mnemonics') involve imagining a standard list of places or items such as a town hall or a prehistoric man. And the memory expert knows his special list perfectly because he has spent many many hours memorising that pre-prepared list. Once that is achieved, you could ask him what the 30th item on the list is and he will know the answer because he really has poured a lot of time into mastering that special list.
Why does he learn the list so well? Because he can imagine images or values at the places in his list and thus memorise those items vividly. This is a difficult concept to grasp. But, if he has a vivid image in his mind of a town hall and he is asked to remember a random item such as a camera, then he can imagine the camera at the town hall. Now while it is true that he never knew that someone was going to ask him to memorise a camera specifically, he did know that whatever item someone was going to ask him to memorise, he would certainly imagine it at the town hall. The town hall is an early item on his pre-prepared mastered list.
So, when he needs to recall the random items which he has been asked to memorise, he will certainly visit his mental town hall image and see the camera there. He will then recall the next item in his pr-prepared list and see whatever other item he has been asked to memorise.
Another tough concept is that numbers are harder to memorise than images; therefore, memory experts spend hours learning special images which equate to particular numbers. So, if a member of the audience calls out "316!", he will not memorise 316; he will memorise an image which he learned long ago is the equivalent of 316. eg. A shut pen where sheep are kept. So, at the town hall, if he recalls a sheep pen then he instantly knows that it means the number 316.
These ideas are strange but essential. So think about them before you continue with the memory course.