Syntax refers to word order, and the way in which it works with grammatical structures. As we are used to hearing things in certain orders, the effect of breaking with normal syntax is to draw attention to what is being said and the way it is said. P J Kavanagh's 'Beyond Decoration' has a speaker who says, rather than "I cannot go out", "Go out I cannot", which - by shifting its syntax - seems to make the impossibility in "cannot" stronger, as well as creating a reversed echo with the second half of that line. The opening of Dylan Thomas' 'A Refusal To Mourn the Death, By Fire, of a Child in London' is hypnotic in part because of its rhythms and rhyming, but also in that its syntax is designed to put such distance between "Never until..." and "..shall I".
Some poets will also deliberately fracture syntax beyond what is considered grammatically correct, which demands a lot of attention, but aims to repay this attention by revealing things that cannot be said within the habits of thought that grammatical language maintains.