So on the higher levels of Buddhist practice we talk about bhāvanā, cultivation, the training of the mind. This training also has various levels. Initially one may train the mind simply to be calm by the practice of samādhi. The aim of samādhi is to focus the mind at one sensation, or object of awareness. The everyday mind is rarely at rest, it's like a monkey, jumping from one impression to another. One moment it has one object of awareness, then in an instant it flies off somewhere else, then somewhere else again. The more it jumps around following sensations, the less it is its own master, and the more it is enslaved by those sensations and caught up in greed, hatred, and delusion.
If we are able to bring our mind to rest on one object, not jumping around after countless sensations, the mind will become manageable. So we must take one particular impression, anything will do that is wholesome and not outright harmful, such as a meditation theme. For example, one object that is quite neutral and doesn't cause the mind to proliferate in unskillful ways is the in and out breathing. Another is the qualities of the Buddha, which is a very good theme for keeping the mind from wandering around.
When the mind rests with a meditation object we don't have to bother with sensations arising from the outside world. Defilements resulting from value judgments about externals do not arise.
This is the most elementary level of meditation practice: concentrating the mind on a harmless object. With the mind coming to rest on that object, we can be said to have accomplished our aim, the mind is in samādhi, being firmly fixed on one object. When talking of samādhi the word ekaggatā (one-pointedness) is used, meaning that the mind rests with one particular object. When it rests on that object it is calm and undistracted. This is samādhi.
Samādhi has just this much as its initial requirement: bringing the mind to a focus at one particular sensation. The defilements are subdued and unable to arise. If the mind goes on to more refined levels of concentration, it may enter the jhānas (absorptions), but no matter how refined the concentration becomes it still retains the same basic qualities -- having one sensation for its object and fixing onto that.