Students in a monolingual class speak the same first language, and will share most aspects of a culture. Monolingual classes can be compared to multilingual ones, where there are a variety of first languages.
Monolingual classes are typically found in schools in the learners' own country.
In the classroom
In a monolingual class there may be more use of L1, which can create problems but can be a useful tool, e.g. translation can be used when necessary. There is also a shared culture, which a teacher can exploit. The learners may have similar learning strategies, and similar problems learning the target language.