In which context?
Open borders within the EU are there to facilitate free movements of people, goods and money.
States usually have tariffs and taxes in place to safeguard their own economies, meaning, they have to control what gets into a country.
This process takes weeks, months. And it takes longer the more you want to disincentivize the import of a certain good.
When you then make trading unions, all of a sudden you have to remove borders, because the main benefit to trading unions is, that you remove all those barriers, save money for both sides, save people time, raise efficiencies, support different business models, ...
Basically within Europe nowadays we dont have much storage capability anymore - because our industries work "hand to mouth", so you import raw materials, and then use them to manufacture your goods as they come in. That makes everyone involved more competitive - because they save costs.
Now - when you've done away with borders for wares and goods, you basically will want to talk about doing the same for your own citizens. Also because of economic reasons, because it solves allocation issues.
(Germany currently needs medical workers for an overaging population, and they are willing to pay. Italy has a financial crisis and high youth unemployment > Italians take a 6 week german course and move to germany to work for their old folks. Business likes this very much.)
Your "outer borders" (at the edges of a trade zone) are pretty much always hard borders. (With people controlling whos transitioning.)
America has open borders between states for example. But you then also have a system that can give money from richer states to poorer states to keep them at least "functioning", or help them out in a crisis f.e. - so basically with open borders you save everyone money and time, and get bigger markets (one ruleset instead of lets say 15 important for exports from other countries > who then are more willing to trade with you, ...) - but you also have to keep everyone roughly at the same economic "competitiveness" level, because if you do not - one state would start to undercut the other, or one state would want to refuse to payback state loans (if they see the system as unfair) - and then the system gets unstable.