Imagine you work in a big supermarket. You work behind the checkout, scanning items that are constantly arriving on the conveyor belt. Each time an item reaches you, you pick it up, quickly scan it, briefly check the short description of the item which appears on your screen before passing it on and moving on to the next. If you spend too long with an item, checking the code or chatting with the customer, then a long line will start to build up behind you. However, if you move quickly and scan the items efficiently, the queue will disappear and you will be left wth an empty conveyor belt. You can then take a break and do something else more interesting rather than sitting behind the checkout scanning items all day.
When meditating, I like to think of the mind like a giant conveyor belt, and your 'self' as the checkout person. Each thought is like an item on the belt and it is your job to sit back and observe each thought that arises, quietly and efficiently. Thoughts will come, sometimes quickly, sometimes slowly. You have no control whatsoever over what these thoughts will be – some will be positive, some will be negative. Some will be exciting, some will be boring. But as a checkout person it is not your job to start discussing whether or not a particular item is good, bad, exciting or boring. Instead, you must stay removed from the stream of items and merely observe each one as it arises, label it, and let it go. One way to do this is to create a short and appropriate description of the thought as it comes in, then let it pass as quickly as it came.
For example, a thought may arise about a new recipe you'd like to try out for dinner. Ordinarily, you would probably begin the process of thinking about this recipe. You would then likely spend five minutes or so thinking about the ingredients you need to buy, the utensils you need to cook it and the timing it will take to prepare. You may even start to think of inviting people to share the meal with you, or worrying that there is an ingredient you won't be able to find. You may even abandon the recipe and start thinking of other recipes you could try instead. Rather than letting yourself be taken on a ride with your thought in this way, you can instead create a label for it, much like you would see flash up at the checkout. For example, this thought could be called 'Thought about new recipe.' That's enough. You can then put it aside and move on.
You don't need to invite new thoughts in. As with the conveyor belt, they will keep coming regardless of whether you want them to or not. In the supermarket, items have many categories – meat, fruit, vegetables etc. Thoughts have three categories: past, present and future. One way you can label your thoughts quickly is by simply identifying their category and then moving on. For example, the thought about the new recipe would be labelled 'Future', as it is a thought about something you would like to do in the future. If a memory crops up about something that happened yesterday, this can be labelled 'Past'. Or if you hear a noise outside or you feel the need to scratch your leg, this can be merely labelled 'Present'. In this example, you do not need to scratch your leg. You will find that if you label the thought 'Want to scratch leg' and move on without scratching it, within moments the 'feeling' will pass and you will have a new thought on your conveyor belt.
This is not to say that in our lives we should not engage with our thoughts. Of course we should. But meditation is the process of training our brain to be witness to our thoughts, not victims of them. If we can train our brains to observe our thoughts regularly instead of rashly reacting to them, we can gain a huge amount of control over our minds. We are able to choose what we engage with and what we don't. We suddenly give ourselves the power to choose to only to engage in positive, useful or important thoughts, rather than wasting time and energy on thoughts that will not benefit our lives - or worse, thoughts which will lead to negative emotions such as anger, fear or hate. By becoming masters of our own minds, we start to become masters of our own lives.