A "Computed Field" is a component that executes custom Javascript code. You can select other components as parameters/arguments for your functions. The code will run when those components' value changes.
Some Examples
Concatenating first_name and last_name, then returning full_name
Returning a different price ID for a stripe component depending on the user's selection/s
Fetching data from/to an API
Returning an array of objects for a repeatable buttons component
