An event property produces a single value for each event.
An event property can exist in your source data, in which case the UI identifies it as a raw event property. Or a user can define a custom event property, in which case the UI identifies it as a manual event property. When you define an event property, you must choose a method.
For example, if your log data includes a start and end time, you could define a new event property using the calculate method, named "duration", as [end time] minus [start time]. You can use "duration" in a more complicated query (in an aggregation, split by, or filter) or to construct another event property. Then the top-level query can compute duration for any event in your data that has both a start and end time.
Building an event property in the User Guides
Create an event property in the Tutorials