Entry Depth (or `entryDepth` when used as a parameter in the SDK) refers to the depth at which Nacelle indexes entries from Contentful. To help clarify, consider the following Contentful models:
Let's consider Blog Post entries. Each post entry exists at layer 0. An entry may have a reference to another entry. For example, the "Welcome to Our Blog" entry may have a reference to a hero. This reference is layer 1, and layer 1 may have a reference to another entry (layer 2), which may have a reference to yet another entry (layer 3), and so on.
By default Nacelle will index three layers deep. The more layers you have, naturally the larger the response will be.