This article will metion what resueable flows are.
When you create a reusable flow, you can use it in your component flows, backend flows or anaother reusable flow.
First, you need to create your reusable flow. if you don't know how to create it, you can learn it from How to Create a Flow
Then go to your flow which you wnat to implement and click the plus icon and Reusable Flow button as you see in the image below.
Lastly, click your added reusable flow node and select your resuseable flow as you see in the image below.
Congratulations! You implemented your reusable flow.