The great way to make them understand is to explain it in the context of their own religious beliefs. Starting it this way will allow them to understand the practice in a familiar and friendly way.
The key to explaining fasting is to say it in a loving way, not derogatorily. If you see that they’re really curious and probe you with more questions, that’s when you can share why you do it.
Lastly, pray that opportunities like these will open their hearts to being more curious about Jesus, and open doors for you to sharing the Gospel with them.