Every language has rules as to what makes a correct sentence. Syntax refers to the correct order of words in a sentence, so that the sentence is grammatically correct, and more importantly, makes sense. Non-native speakers of English may experience the greatest challenges mastering English language syntax.
A simple sentence has a subject, a verb and an object, in that order. The sentence "I saw Jim," shows correct syntax, whereas "I Jim saw" does not. As native English speakers acquire more advanced writing skills, subject-verb and subject-verb-object agreement become important. So a sentence like "you were dancing" shows correct syntax, but a sentence like "you was dancing," does not. A sentence like "a customer should deposit her check in the bank" shows correct syntax, whereas a sentence like "a customer should deposit their check in the bank" does not.