When you get an Aadhaar card, you also give the government your fingerprint and retinal scan data. This is called biometric data. Your biometric data can be used for Aadhaar verification, which means that (for example) when you submit an Aadhaar card as a proof of identity to get a SIM card, you can use your fingerprint to allow the telecom company to access your identity information. By doing this, it can quickly authenticate the ID, speeding up things like KYC verification.
However, there have been a couple of cases where people have claimed misuse of Aadhaar biometric authentication. People who haven't used their Aadhaar card for a while have received an email from UIDAI stating that their data has been accessed via biometric authentication. This is obviously quite alarming.
All this can be avoided if you lock your biometric information stored on UIDAI's servers. This would mean that the biometric data can not be accessed by anyone else - you can unlock it whenever you need to use it for verification, and then lock it again.
ALSO see How to Check Aadhaar Card Status Online. Here's how to lock and unlock your Aadhar UIDAI biometric information.