Some Actions are editable so you can express how many times you have done it. For example, with using a reusable water bottle, you can edit your Log to say how many times you refilled it. This prevents you from having to Log it every time you do a small task.
First, find the Action you want to Log and tap on it. Then tap "Log Action". If the Action's quantity is editable, there will be an 'Edit' button in the top right corner of your screen. You can then select the quantity of what you have done.