These are pretty words that want you to have a good impression of OpenStack. They don't mean much, and can be interpreted in several ways. Here is my interpretation, if you are interested:

Modular architecture: It consists of modules (Nova, Cinder, Neutron and so on) that you can choose to deploy or not deploy. For example, you may decide not to deploy Cinder.

Scalability: When you find out that your cloud's resources are exhausted, you can add resources like compute nodes or storage systems easily.

Elasticity: Cloud applications can grow and shrink depending on demand and only consume the resources they need at any given point in time. This is actually a main characteristic of any cloud, so it's fairly pointless to brag that OpenStack supports elasticity. It's like saying that a BMW allows you to drive.