A feature of syntax - the order in which words go in a sentence (phrase) in order to create meaning.

For example, English has a fairly strict Subject-Verb-Object word order. This means that phrases are generally structured in this way:

subject / verb / object

who / does what / to whom

Mary / hit / Julia

If we changed the word order so that the object were in front and the subject were at the back, the original meaning would be lost:

Julia / hit / Mary.

In English, the subject must always precede the object (passive sentences are the way of getting around this rule), because there is no other way of telling who did what to whom.