What are syntax rules?

Syntax is the discipline that examines the rules of a language that dictate how the various parts of sentences go together. While morphology looks at how the smallest linguistic unit (called morphemes) are formed into complete Words, syntax looks at how those Words are formed into complete sentences. Syntax is not prescriptivist - which is to say, it does not attempt to tell people what the objectively correct way to form a sentence is. Rather, it is descriptivist, in that it looks at how language is actually used and tries to come up with rules that successfully describe what various language communities consider to be grammatical or non-grammatical. Syntax deals with a number of things, all of which help to facilitate being understood and understanding language. Without rules of syntax, there would be no foundation from which to try to discern meaning from a bunch of Words strung together, whereas with syntax, an infinite number of sentences are possible using a fairly small finite number of rules.
to put it simply syntax is the arrangement of Words in a sentence or paragraph ect...

but the exact dictionary definition is:the study of the patterns of formation of sentences and phrases from Words.

Syntax is the general rule that humans have to abide by which is that the composition of a coherent sentence using grammar and structure must be cognitively structured in the mind of someone's brain before they can communicate it to a receiver through means of making a series of small mouth noises. Syntax is the arrangement of Words and phrases to create a message in a language.