Observe field is the most important part of brightscript language ;-
We can use observe field in several places like-

1 ) In any Interface field we can use observe field like when data come then callback function will run.
2 ) In any button when we click on that button then callback function will run .
3 ) In every xml element group, button and all we can use observe field.

In Roku development, the observeField function is used to monitor changes in the value of a field within a component's associative array. This is particularly useful in scenarios where you want to update the UI or perform certain actions based on changes to specific data.

https://erpsolutions.oodles.io/developer-blogs/Observe-Field-in-roku/