# Core concepts
This section defines the core concepts of MeiliSearch:
- Documents: Objects made up of key-value pairs
- Indexes: Entities that contain documents and search-related settings
- Relevancy: Rules that help you get more intuitive results. The human element of your search engine
MeiliSearch is an asynchronous API. This means that, unlike a synchronous API, it doesn't wait for a call to finish before reporting the results and accepting a new call request. Instead, calls are placed in a queue for processing. For further information please read the dedicated guide on asynchronous operations.