This explanation is quoted from my article about Signal
Metadata leaks are when your messaging app includes metadata in a way that's not encrypted end-to-end. This includes the dates and times you send messages, who you send them to, how frequently you send messages, and more. Many messengers allow you to set status messages and profile pictures. These are not encrypted, and are thus leaked. The issue with metadata leaks is they can fairly easily reveal sensitive information, even when you don't expect them to.