Metadata is all the content of a message besides the actual text content of the message (i.e. what you type). Examples would be the date and time it is sent, what users these messages were sent to / from, and the IP addresses of both parties. (The availability of metadata varies from messenger to messenger).

I like this example: If you only text your Aunt Sally, who lives in Alaska, twice per year to wish her a happy birthday and Christmas, just by looking at the metadata someone could infer the meaning of your messages, as well as your relationship to the person you're messaging. To a point this is true about any messages you sent.

As for Whatsapp specifically, it being end-to-end doesn't really matter imo, as the application is not open source and is owned by an advertising / social media company. As long as the code is closed source, you cannot be sure:

1. That your messages are encrypted at all
2. That your encryption keys are kept on-device, and not plainly available to a centralized party
3. That the encryption the application is using is securely implemented

At least for applications handling truly sensitive information (for the average person only their messenger and browser), you should be using open source software. The easiest recommendations I can make are:

1. Browsers: Firefox, Thorium, Brave (disabled all cryptocrap)
2. Messengers: Signal, SimpleX Chat, XMPP

Anyways, I hope this was a satisfactory answer.