// The ArrayList
List<String> mods = new ArrayList<>();

// Saving it to the config
getConfig().set("mods", mods);
// "mods" is the path you are saving it to and mods is the ArrayList

// Saving the config, always do this when changing something in the config
saveConfig();

// Retrieving it from the config
mods = new ArrayList<>();
// Empty mods, or create the variable again

for (Object object : getConfig.getList("mods")) {
	mods.add(object.toString());
}

// Now all the players in the list from the config are in the ArrayList, mods.

// Now that is how you use it!