Explicit knowledge is characterized by the fact that it can be easily captured in some form: a book, a course, a seminar, or any other medium. It is the most basic form of knowledge, and the one we usually think about when imagining expertise.
For example, if I’m studying a new programming language, the language’s syntax rules can always be found in a manual.
This is in contrast with tacit knowledge, which is hard to capture.
The result is that, when learning, you typically start from explicit and move to tacit.