Our practice of Zazen within the Soto Zen tradition is also called shikantaza, which is generally rendered in English as ‘just sitting’.
How we tend to interpret the phrase is that we should sit wholeheartedly, without expectation of gain or achievement. It’s a psychological explanation. The contrast is then with other forms of meditation which we say are goal directed.
Within Zen, the classic contrast is with the koan practice of Rinzai Zen.
Soto people will say this is goal directed, because it is concerned with attaining kensho (a visceral experience of sudden awakening).
The actual Rinzai position is much more nuanced than this simple contrast. They tend to have a similar caricature of Soto.
This understanding of ‘just sitting’ is unfortunately an error, one which is very common when translating technical terms from one language to another.
The primary meaning of ‘just sitting’ is in terms of non-separation. In other words, when I’m sitting there aren’t two things: a self and a world; there is simply this whole, this ‘one piece zen’ into which, as it were, both the self and the world have disappeared into.
Obviously, from that perspective, there is no expectation of gain or achievement, because there is nothing distinct for either to adhere to, but the phrase is experiential and descriptive, not psychological.