What is the From/since Parameter?
The "From/since" field allows your recipe to go back in time to pick up history objects that meets the trigger condition up until the present time and move on from there. This means that if you set it to "an hour ago", it will pick up events that occurred from an hour ago, and will continue picking up new jobs created afterwards. You are either able to choose from the dropdown fields for certain apps, or you can click on the toggle and choose a specified date and time by clicking on the calendar logo on the left of the field.
DO NOTE: The since parameter will only work for the first time you run a recipe. After the first job has been processed, the recipe default behavior is to pick up where it left off. You will not be able to make changes the field after your recipe starts. If you want to make your recipe go back further in time to pick up older jobs, simply make a copy of the recipe if you would like to make changes to the look back date/time. Be careful when doing so as you might end up with duplicates records, unless you set the trigger filter to not pick up records after a certain date. Check out this article on how to prevent data duplication.
Next: Understanding the Trigger: Trigger FIlters - How to reduce Transaction Count