The best way to learn how to use ReActions is creating your first simple activator.
Here is step-by-step instruction of activator that will teleport you in predefined location when after pressing a button.
|Place button on the block. Point your cursor on this button and type command: ||Creating a new button-activator.|
|Now you must move to target location and type command||Creating a teleport (TP) action linked to activator tpbut.|
|Move back to your button and press it.||Congratulations! Your first activator successfully created!|
This activator will be saved in
activator.yml file. And you can edit this file manually:
BUTTON: tpbut: world: world x: 1355 y: 80 z: -2122 flags:  actions: - TP=world,1355.389,84.0,-2113.658,180.884,36.6 reactions: 
You can find more ReActions tutorials in this playlist.