The Start Reactor Task in Polus is a long task also known as 'SimonSays'. Once the task is started, a screen will appear on the left while a keypad appears on the right. You need to memorize the order blue squares appear on the left screen and then replicate it using the right keypad
It will start with memorizing 1 pattern up to 5. Don't worry though, it's not as hard as it sounds. The patterns will always repeat and build on each other. For example, the second pattern will always start with the blue square from the first pattern.