This means that every so often the item would do something.
For example, an item that says in its description: “Once in a while, has a 30% chance of restoring X amount of health (plus a bonus due to chemical knowledge) ally’s health.”, this will significate that it has a 30% of chance that it tries to cure the soldier, but if it fails, it will try again, that’s why it’s “once in a while”.

This will be invisible in battle, but you can be sure it may happen due to the item’s nature.