The "declaration merging" means that the compiler merges two separate declarations declared with the same name into a single definition. This merged definition has the features of both of the original declarations. Any number of declarations can be merged; it's not limited to just two declarations.

Basic Concepts

In TypeScript, a declaration creates entities in at least one the three groups: namespace, type, or value. Namespace-creating declarations create a namespace, which contains names that are accessed using a dotted notation. Type-creating declarations do just that: they create a type that is visible with the declared shape and bound to the given name. Lastly, value-creating declarations create values that are visible in the output JavaScript.
Declaration Type

Merging Interfaces

The simplest, and perhaps most common, type of declaration merging is interface merging. At the most basic level, the merge mechanically joins the members of both declarations into a single interface with the same name.