I bought a used iPhone 11 and checked the iCloud is clean but if I check online it shows activation Lock is on please what does that mean?

I am not familiar with what it means when you say iCloud is clean.

'Activation Lock for iPhone, iPad, and iPod touch' is a security feature that's designed to prevent anyone else from using an iPhone, iPad, or iPod touch, if it's ever lost or stolen. To learn more click here --> http://support.apple.com/HT201365

For an individual to remove an activation lock absolutely requires the information for the account that applied the lock. Click on this link to learn 'How to remove Activation Lock' --> https://support.apple.com/HT201441

If you do not know the required information and you are not the original owner, get the owner of the account to remove the lock or return the device for a refund or recycle it. The device is otherwise useless to you. If the owner is unknown and has not provided contact information as a message showing in the login screen, there is no way to contact the owner unless the person who gave you the device knows them.

I am guessing that is some third party service with which I am completely unfamiliar, never heard of before, and would not have a lot of faith in versus what it says on a device itself. It's what the phone itself thinks or an Apple service displays that counts in this case.

What Apple does not track is if somebody has blacklisted a device with its IMEI on a blacklist service with phone companies. That is not an Activation lock feature and not anything to do with Apple.