What is Open Next File in Folder?
Open next file in folder is a functionality found in applications that open files such as image viewers and video players in which upon opening a single file from a folder it's possible to navigate from the application to the next file in the same folder without having to manually add all files in the folder to the application.
Opening the next file in folder isn't the same thing as being capable of opening a folder. An application may be able to open a folder (e.g. by dragging and dropping the whole folder into the application) and in doing so it will automatically find all files in that folder that it supports and load it into a list for display. This is different from simply being able to open the next file in folder for two reasons:
First: when opening a whole folder, the application decides which file is the first file to display through some sorting algorithm. Most likely, the files will be sorted in alphabetical order and the first so the first file in alphabetical order will be displayed when you open a folder. By contrast, when an application is capable of opening the next file in a folder the user can choose which file should be displayed first by selecting it from the file manager.
Second: when opening a whole folder, if the application merely automates the creation of a playlist or something similar, then changes to the folder's contents after the folder has been opened won't be noticed by the application. That is, when you open a folder, you're merely creating a list of filepaths that exist in that folder at that moment. If you add a new file to that folder or delete a file while the application is open, the list of filepaths in the application won't change, so it won't be able to display newly added files and it may even error when trying to display files that were deleted after opening the folder. By contrast, an application that can open the next file in folder is likely to try to find the next file in the folder the moment the user presses a button instead of generating a list of siblings when a file is opening. It's worth noting that both approaches have valid use cases, as generating the list only once allows the application to switch from one file to another faster as it doesn't need to gather all files and sort them every time you click the next file button.
Examples
Most image viewers can open the next file in folder.
Some video players like MPC-BE can also open the next file in folder.