Open/close can refer to the price of a particular currency (or stock) at the time it comes on the market, and when the market closes. These terms are often more helpful in traditional financial markets because trading takes place at set times throughout the day, whereas cryptocurrencies do not have this type of trading process.
Open/close also can refer to the programming paradigm of expandable software components (functions, modules, etc.) without enabling any form of source code modification in software development. While limiting the ability to alter its core elements, the code itself allows for flexibility.