It refers to Entity-Component-System, an architectural design pattern (like Model-View-Controller) that uses composition instead of inheritance to define the behavior of entities in a simulation.
What they are referring to as ECS is more than just composition over inheritance. There are entity component architectures that are just that--Unity's existing class based component architecture for example.
But the ECS in this case refers to Data Oriented Design--basically laying out data like normalized database tables. Every component system has a table, and each row in the table is the data for one component (you don't actually use a relational DB for this, the data is just organized in a similar manner). The entity itself is just an id that all of the components reference.
The concept as used in games started in the late 90s as Structs of Arrays instead of Arrays of Structs.
Here's a a good free book on the subject (there's a paid hard copy as well) http://www.dataorienteddesign.com/dodmain/