In plain English
CI means continuous integration: developers regularly combine their changes, with automated builds and tests checking the result. CD can mean continuous delivery or continuous deployment. Delivery keeps changes ready to release and can include human release approval. Deployment automatically sends changes to production when the required checks pass.
Example
A startup's pipeline tests each code change and prepares a release. A team member approves sending it to customers. That workflow uses continuous delivery; automatically deploying every change that passes the required checks would be continuous deployment.