It gives the original author the right to relicense your contributions at a moment's notice, to a license you might not agree with. It effectively means that you cannot contribute to the project under fair terms.

A CLA is similar to a EULA in the sense that most people don't care about it and blindly click "Agree", and as such are unaware of the potential consequences. Look at what happened when HashiCorp relicensed its projects. People were outraged. They agreed to the CLA, so they must have known that this could happen. But did they really take the time to understand what they were signing?

As a contributor, you should never agree to a CLA. As the original author, requiring your contributors to sign a CLA sets an extremely bad precedent.