A syllabus is a document that describes what the contents of a language course will be and the order in which they will be taught. The content of a syllabus normally reflects certain beliefs about language and language learning.

Example
A syllabus might be designed around the order in which grammatical items are introduced. Starting with 'present simple' then 'past simple', then 'present perfect' etc.

In the classroom
There are many different types of syllabus (although often in language classrooms the syllabus from the course book is the only document). Syllabus types include grammatical, lexical and functional, which focus on the building blocks of language, and task-based and learner-centred, which focus on processes of communication and learning.

See also:
https://www.teachingenglish.org.uk/article/grammatical-syllabus
https://www.teachingenglish.org.uk/article/process-oriented-syllabus
https://www.teachingenglish.org.uk/article/product-oriented-syllabus
https://www.teachingenglish.org.uk/article/task-based-syllabus

Further links:
https://www.teachingenglish.org.uk/article/which-syllabus-traditional-holistic-syllabus
https://www.teachingenglish.org.uk/article/syllabus-writing
https://www.teachingenglish.org.uk/article/general-english-syllabus-design
https://www.teachingenglish.org.uk/article/designing-business-english-programmes-2
https://www.teachingenglish.org.uk/article/putting-eap-practice
https://www.teachingenglish.org.uk/blogs/dario-banegas/literature-content-based-instruction
https://www.teachingenglish.org.uk/blogs/vicky-saumell/vicky-saumell-coursebooks-guides