as factor says, if you want to move house and you already have a house, it becomes necessary for you to buy a house and sell your house at the same time (if you were to buy a new house before you'd sold the current one, you'd be paying for 2 houses - if you sold the current one before you'd bought a new one, you'd have no-where to live)
So, you find a house you want to buy and the people who live there already have found somewhere they want to buy, and that place they want to buy, the people living there have found somewhere they want to buy. 4people in a chain already.
it is possible to move without a chain, but only if you are lucky and, for example you are a first time buyer buying the house of someone who is not buying somewhere else