Discussion on: What’s a concept you understand now, but took you forever to grasp?

1. JavaScript Promises.
2. The referencing of this in JavaScript.
3. CSS display property. (Sometimes I still don't get it).
4. Lambda and Proc in Ruby.
5. DyanmoDB
6. ECS and EC2 on AWS. (I still don't get it).

This list could cross the count of a hundred.