A markup language divides a text document into chunks and describes what those chunks are supposed to be. This meaning can be read and interpreted by software programs, making the text far more useful. Ideally the language will be set out in a way which is easy for humans to understand as well.
In web development, the most common markup languages is HTML. All web pages are written in HTML. Another popular markup language is XML which is used to transport data and develop other markup languages.
Read more about markup languages at these resources: