How does my system understand if data got masked?

I was trying to understand the difference between encryption and masking Below statement says that real data replaced and gone! >Masking protects your data by transforming it into a readable format that’s useless to anyone who steals it. The actual data is replaced by fictional information. There is no encryption solution to revert the data to its original state. The real data was replaced and is gone forever. What does it mean? If real data are gone, then how do I use it in my system? **For example:** I might masked EMAILID and my system wants to do some logic based on the email ID, then how do I compare the email ID in code? Basically, if I am not using the sensitive data, why do I store it in my database in the first place?