The rules for how words can be combined to form sentences in a language.
When these rules are not followed, it is sometimes possible to understand what a speaker/writer means in spite of syntax errors, but it doesn’t sound right to native speakers. For example, consider the following sentence: “I rode this morning the AppalCart to campus.” This is an example of a syntax error of word order in English.
Other languages do not necessarily use word order to create meaning; some are inflected, meaning that the forms of words change (by adding or changing a prefix or suffix) to define to how the words are used correctly.
For more information on the five domains of language, see the American Speech-Language-Hearing Association.