Quick reaction alert is a special military practice where airplanes and soldiers are kept ready to act very quickly in case of an emergency. Imagine you are playing a game of hide and seek with your friends, and you want to make sure you can find them really quick if they need your help. So, you stay very alert and ready to jump into action if your friends call out to you.
The military does something similar to protect its country. They have special groups of soldiers and airplanes ready to protect their people from any possible danger. These soldiers stay awake and alert all the time, even at night, to make sure they can move quickly and respond to an emergency. Just like you would be ready to help your friends in a game, these soldiers are ready to protect their country in real life.