The medium of instruction is the language used by the teacher to teach. Teaching the language, or educational content, through the target language increases the amount of exposure the learner gets to it, and the opportunities they have to communicate in it, and therefore to develop their control of it.
English is used from the beginning of a course as the main language in class, and the teacher adapts their methodology to support meaning, by using a lot of visual information and non-verbal communication to support meaning.
In the classroom
The question of which medium of instruction to use, and the impact on learners and learning, is a complex one. It is useful to consider how the language used supports the aims of the class. For example, it may fit the aims to give all classroom instructions in English in a low level class, because this is useful exposure to new language and will be learnt, but explaining the methodology behind an activity to the same group might be done in L1.