Values that change based on participant input or through other dynamic events in the experiment are called variables.
Variables are universally marked with "%"s around the variable name in Testable (e.g. %response%). When the experiment runs, the whole variable (including the %s) will be replaced by the value it represents.
You want to show your participant their reaction time from the previous trial on a feedback screen:
This would be your trial file snippet:
Your reaction time was %RT% ms.
Example outcome in experiment (at the feedback screen):
"Your reaction time was 768 ms."
The variable was replaced by the actual reaction time in the previous trial. Read on to learn about the different types of variables that you can use.