WAT is an entity linker, namely a tool that identifies meaningful substrings (called "spots") in an unstructured English text and link each of them to the unambiguous entity (an item in a knowledge base). Entities are Wikipedia/Wikidata items. This has applications in a range of NLP/NLU problems such as question answering, knowledge base population, text classification, etc. You can annotate a text by issuing a query to the RESTful API