Free word order is a thing about which there is some conceptual confusion among conlangers. There are essentially two ways in which a language can have free word order.
First, there's the more obvious meaning of it: a language that has this kind of free word order permits reordering the constituents of a clause freely.
Second, there's the less obvious meaning of it: a language where such reordering does not change the meaning of a statement.
The latter meaning might require some elucidation. Clearly, English does not have free word order, as the following couple of sentences illustrate:
a) Bill hit Pete
b) Pete hit Bill
c) *Bill Pete hit
d) *Pete Bill hit
e) *hit Bill Pete
f) *hit Pete Bill
A) and b) are otherwise rather similar, but their meaning is different – in a), Bill carries out an action and Pete bears the brunt of it, and vice versa in b). Clearly, English restricts word order fairly much, and even the two possible reorderings of {Bill, Pete, hit} have a very distinct meaning.
In some languages, reordering the arguments can change things like topicality, new information vs. old information, etc. One possibility could be that
Bill hit Pete
simply says that Bill hit Pete, whereas
Bill Pete hit
serves to specify the object of the hitting, i.e. Bill hit Pete, not anyone else in particular: "It was Pete that Bill hit" . Other similar changes of meaning are possible:
hit Bill Pete
in most Germanic languages this is how you ask if Bill hit Pete!
In some languages, pretty much each reordering has just a slight change of nuance, often with regards to information structure. Some of these languages obviously mark objects and subjects either by congruence on the verb or by case markers on the nouns. However, some have other approaches as well - we may, for instance, have contextual knowledge. Stones don't eat children, but children do at times eat small stones, etc. This might permit for utterances like "the stone ate the kid" with the meaning "the kid ate the stone" in a language that does not mark case nor subject and object congruence on the verb.
