Syntax is a form of grammar. It is concerned primarily with word order in a sentence and with the agreement of words when they are used together. So it is, in a sense, acting as a kind of ‘police officer’ for the way in which sentences are constructed.
English is a language that has a structure known as SVO. That is subject, verb and object. The cat (subject) washes (verb) its paw (object). This is the correct word order and also there is agreement between the words.
If there were no agreement within the sentence, it could read, “The cat washes their paw”. This does not make sense. The cat may have four paws, but it is only washing one paw. For there to be agreement, the possessive ‘it’ has to be correct. Thus “The cats (plural) wash their (plural) paws (plural)”. This is the correct use of the plural possessive (their).
At first, syntax can seem daunting and it is always difficult initially to understand what a ‘subject’, ‘verb’ or ‘object’ actually is. It can also be difficult to understand whether agreement between the ‘subject’, ‘verb’ or ‘object’ is right or wrong. There are lots of tools such as grammar checkers, programmes or worksheets to help you get to grips with syntax and to make sure that you have the right word order and that within the sentence there is always agreement between the words, tenses and so on.
It is true that syntax can take some time to master, but, once you understand its principles and can apply it without too much effort, then it really is worth the effort, since it will greatly improve your written English.